Freddie the computer science student had to go cold turkey last year. When Spud the philosopher quits for a week, he feels "very strange." Julie the theater historian says, "It's my hallucinogen of choice. . . . I love being able to slip into another body, another persona, another world."

All describe themselves as heavy users. Not of drugs, but of the games, chat lines, news groups, databases and other services on the Internet, that invisible digital landscape created by thousands of linked computers from Stockholm to Sydney.

These committed cybernauts have spent 40 or more hours a week on line, have friends they know only by Internet nicknames and sometimes feel that time on the net is the most rewarding part of their day. They are, as one researcher put it, "really living parallel lives."

Some fear that marathon netters risk disconnection from "RL," the cyber-name for real life. But one faction of psychologists and sociologists disagrees, saying the net is a relatively healthy place for people to deal with deeply rooted problems.

"People are working through something or escaping from something or finding something important for them," said Sherry R. Turkle, a sociologist and professor at the Massachusetts Institute of Technology, who is writing a book about about how people act in simulated environments.

Freddie has been an undergradu- ate at the University of Maryland Baltimore County for five years, but is still a sophomore. He blames the delay, in part, on what may be the most addictive activity on the net -- MUDing.

"There are some things I really get into and all else just disappears," said the 21-year-old, who asked that his real name not be used.

MUD stands for Multiple User Dungeons or Dimensions, and the technology is simple: Computers create imaginary castles or mansions or landscapes through simple word descriptions.

A room might be created, for example, with the words, "You are in a large dark room" or "You are in a closet stuffed with toys. There are 22 other people jammed in with you." Players from all over the world plug in through the Internet, adopt a nickname and type their conversation and actions.

They use simple commands such as "go" to move between rooms or "say" to talk. Other players in the same room or space generally can read those words and type back.

It may not sound fascinating, but MUD players may be among the most obsessive on-line users. Some players say they spend 40, 50 or even 80 hours a week playing, stealing time from their jobs or their classes, forgetting meals, skipping classes, losing sleep.

Over one two-year period, Freddie said he spent the equivalent of 42 24-hour days playing a single MUD adventure game, based in a computer in Sweden. Once he spent 36 hours straight MUDing in at a school computer, without eating or sleeping, pausing only to go to the restroom.

"I was hooked, addicted," he said.

By last fall, his net life began to slop over into RL. In MUDs, players express their emotions -- such as a smile, a grin or a giggle -- by typing them. "I was sitting in my kitchen with my mom and she said something, and I said 'Smile,' " he recalled. "I thought, 'I'm not supposed to say it. I'm supposed to do it.' "

A bit shaken, he quit for more than two months. "I have not been that wrapped up in it since," he said. Now he is part of a team at UMBC designing and programming an educational MUD for teaching classical Greek mythology. At most, he MUDs himself a few hours a day.

Freddie does not regret his past addiction. A shy kid who was good at mathematics, Freddie grew up in Westminster, where few others shared his interests. "I was not well-liked," he said. "I really didn't have any friends there."

In cyberspace, he has immediate access to a cadre of friends, including "ppannion," "jimm," and "therix" -- three players he considers pals, even though he doesn't know their real names.

Learned to express self

Freddie said that, while it interfered with classes, MUDing helped him hone his computer skills. He also learned to express emotions without fear of ridicule or reprisal. "When you're with people face-to-face, you're wearing a mask," he said, echoing the views of several other MUDers. "

In a MUD, everyone uses a nickname or "nick." "You get away with more," he said. Inhibitions are often checked at the virtual reality cloakroom. "In the physical universe, you wouldn't go out there and say, 'Hey, you want to have sex?' " said Freddie. "But on a MUD, you can do that. And you can go off somewhere and do your text." (On the Internet, sex is basically dirty typing.)

Dr. Turkle said MUDs offer a rare chance for people to experiment with different personalities.

"The ability to play another persona, another aspect of the self is one of the most riveting things about the Internet," she said. "Some of them are handicapped, some of them are not pretty people in their own eyes or the eyes of others. They don't feel as presentable in person as their verbal skills and charms and wit -- a too simplistic analogy is to Cyrano -- as they are on-line.

"What part of ourselves would we allow to come out if we were as beautiful as Julia Roberts?" she said. "Now people are having the experience of who they would be if they could just be their words. It's a very heady experience."

Larry Rosen, a professor of psychology at California State University at Dominguez Hills, sees peril where Dr. Turkle sees potential. "You lose track of your normal social relations," Dr. Rosen said.

"Instead of talking to people, talking to friends, visiting, you're on the computer all the time. It creates this artificial feeling of closeness, whether you're on a game or on some bulletin board or what, that you know them. But in fact it's not real." People, he said, are "deluded" into thinking they have friends.

Few formal studies have been conducted. But a University of Nottingham researcher, in a study published in 1991, found that 75 "computer-dependent" subjects "had logical reasons for choosing to turn to interaction with a machine for the satisfaction of their needs." She concluded that heavy computer use provides "an admirable means of coping for those who may have previously felt inadequately fulfilled."

Andrew Henderson, a 27-year-old computer specialist with a Baltimore publisher, is a regular player on a MUD called LambdaMOO, where he met his current girlfriend. Some other netters, he said, are troubled by the question of whether their net relationships are meaningful.

"As far as I'm concerned, I consider the relationships I have on the MOO entirely real," said Mr. Henderson, who has a bachelor's degree in philosophy from Indiana University. "If I have a friend on the net, it's a real friend. There's no caveat to it. The truth value of the relationship is the same as any relationship."

In cyberspace, he asserted, people get to know each other on a deeper level than is possible -- at least at first -- face-to-face.

'Closer relationships quickly'

Julie Krute of Champaign, Ill., a heavy MUDer interviewed by e-mail, said "the intense nature of the conversations lead to very close communication. . . . Spending hours and hours per day with someone doing nothing but talking is certain to lead to closer relationships quickly, no matter what the forum."

Ms. Krute, 19, a graduate student in theater history, said she plays computer games six to eight hours a day -- and last summer spent 12 to 14 hours daily on the net.

"My best friend is someone I have known over the net for two years now, but we are meeting in RL for the first time next week!" she wrote. "Netting has both hurt and helped my social life. . . . I have no life so I MUSH (a form of MUDing), but because I MUSH I don't get out and get a life. It's true that I don't leave my house much, but I have a much wider circle of friends now than ever before . . . closer friendships, too."