The General Assembly moved Saturday to dramatically change Maryland's drug laws as the House of Delegates joined the Senate in voting to make possession of small amounts of marijuana a civil offense punishable only by a fine.

The House voted 78-55 to approve a measure substantially similar to the decriminalization bill overwhelmingly passed by the Senate last month. Senate President Thomas V. Mike Miller said his chamber would agree to the House's relatively minor changes and send the bill to Gov. Martin O'Malley.

Meanwhile, key lawmakers also have reached a tentative compromise on how to revamp the state's medical marijuana law to make sure patients can get the drug. That measure is also expected to receive final approval before the Assembly adjourns at midnight Monday.

House Speaker Michael E. Busch, who came into the session a skeptic about treating marijuana possession as a civil offense, cast his vote for decriminalization.

"The fact of the matter is a lot of people utilize this, and it has not been effective putting them in jail and throwing away the key," he said.

The House vote came on a busy Saturday as both chambers scrambled to resolve the remaining sticky issues facing the legislature before Monday night. The tally split largely along party lines, with three Republicans joining 75 Democrats in supporting the measure. Twenty Democrats and 35 Republicans voted no.

O'Malley, like Busch, has long had reservations about more lenient marijuana laws, but the governor has said he will keep an open mind in reviewing the bill.

If O'Malley signs the measure, Maryland would become the 18th state to stop treating simple possession as a criminal offense, according to the Marijuana Policy Project. Two of them, Colorado and Washington, have legalized the regulated sale of the drug for recreational use.

The Maryland decriminalization legislation defines a small amount as 10 grams, a smaller amount than in most states.

The House acted on marijuana after the Judiciary Committee voted 13-8 earlier in the day to reverse its previous decision to approve only setting up a task force to study changes to marijuana laws.

The committee's reconsideration was the result of negotiations between Judiciary Committee Chairman Joseph F. Vallario Jr., a proponent of the task force approach, and a group of his fellow Democrats who want to stop treating marijuana possession as a crime. They include members of the Legislative Black Caucus who point to racial disparities in the way marijuana laws have been enforced.

The resulting bargain gave decriminalization proponents most of what they wanted. Where marijuana possession is currently treated as a misdemeanor that carries a maximum 90-day jail term, the legislation would change it to a civil offense for adults, punishable by escalating fines. Minors caught with the drug would continue to be handled by the juvenile justice system.

The House kept the Senate's $100 fine for a first offense but increased the penalties for subsequent violations to $250 for a second and $500 for a third. For a first or second offense, a violator older than 21 would be able to prepay the fine by mail, but a three-time loser would have to go to court. If convicted, the judge would have to order the defendant into an education or treatment program.

The House took a stricter approach for adults who aren't yet 21 because of concerns about the impact of marijuana on developing brains. The bill approved Saturday would require that people caught with pot when they are 18 to 20 years old be summoned to court and referred to drug treatment or education. In some cases that would involve watching a video about the dangers of taking drugs.

The House and Senate agreed that people found guilty of marijuana possession should not have criminal convictions on their record. Both versions say citations for possession will not be part of the public record and will not be posted on the Maryland Judicial website along with other offenses.

The bill's approval came after an emotional debate in which opponents accused decriminalization supporters of exposing young people to the dangers of drug addiction.

Del. Michael A. McDermott, an Eastern Shore Republican, charged that the bill represented "surrender."

"There's no turning back from the generation of young people you're going to leave behind because you said it's OK," he said.

Some proponents said they considered the measure a civil rights issue because of the impact of marijuana arrests on the future prospects of young African-Americans.

"We are sending a message we aren't going to allow small amounts of marijuana possession to ruin the lives of young people," said Del. Keiffer J. Mitchell, a Baltimore Democrat who took a leading role in the negotiations.