DEMOCRATIC Sen. Edward Kennedy of Massachusetts reaches a milestone today that only four other senators in the institution's 213-plus years have passed: 40 years of service. And he's probably thinking, "You ain't seen nothing yet."

The four were the late Sens. Carl Hayden, an Arizona Democrat(1927-1969) and John Stennis, a Mississippi Democrat (1947-1989), Republican Strom Thurmond of South Carolina (1954-2002 or 2003, depending on whether he resigns before his term officially ends in January, to give his replacement extra seniority) and Democrat Robert Byrd of West Virginia (1959- ).

The senator whom Mr. Kennedy replaced in 1962 was a seat-warmer appointee who resigned the day after Mr. Kennedy was elected to serve out the rest of the term of his brother, President John F. Kennedy. That made him senior forever to Hawaii's Sen. Daniel Inouye, who was also elected that Nov. 6 but took the oath Jan. 3, 1963.

Senator Thurmond, the oldest man ever elected to the Senate at 93-plus, holds the record for Senate seniority. Mr. Hayden and Mr. Stennis served 41-plus years each; Senator Byrd has served nearly 44 years, and Mr. Thurmond will have served 47 years and five months if he stays in office until January. Mr. Byrd has an excellent chance of breaking Mr. Thurmond's record. He will have served 48 years by the end of his current term. And Senator Kennedy has a good chance of beating whatever record Senator Byrd sets.

Both men seem determined to serve well into the future. Mr. Byrd is often asked if at his age he shouldn't be considering retirement. Running for re-election in 2000, he dismissed questions about his age (almost 83 then) by pointing out that Old Testament figures were active well into old age. A favorite example was Abraham, who lived to be 175 years old.

Mr. Kennedy showed his determination Feb. 22, at his 70th birthday party.

"I could run three more times and still not be as old as Strom Thurmond," he said.

If he is elected just one more time and serves a full six-year term, he will pass the 50-year milestone. That alone may not be enough to make him the all-time No. 1. Senator Byrd could also be re-elected in 2006, the next election for both of them.

Mr. Kennedy will be 74 in 2006 and Mr. Byrd 89. Thirty-five senators have been re-elected when over the age of 75. So running at 74 or 89 is not exactly uncharted waters. Barring lethal surprises, they both will probably win: In 2000, Mr. Kennedy got 73 percent of the vote and Mr. Byrd 78 percent.

Mr. Kennedy's age advantage puts time on his side to eventually become all-time No. 1 in Senate service. That would bump Mr. Thurmond to third.

Mr. Thurmond does share one endurance record that is not likely to be broken. He and the other senator from South Carolina, Democrat Ernest Hollings, have represented their state together for nearly 36 years. (Mr. Hollings holds the record for being a "junior senator.")

"Senate" comes from the same root as "senior." It originally referred to a body of Roman elders. They were elected for life. But the senator-for-life kind of career a la Mr. Thurmond, Mr. Byrd, Mr. Kennedy and Mr. Inouye is a 20th century development. There have been just over 1,850 senators. Only 123 served 24 years or more. Only 18 of those served all or most of their tenure before 1900. Only two did before the Civil War.

When I read last winter about Senator Kennedy's teasing hint of his intentions at his birthday party, I recalled his answer to me in 1975 when I asked if his long-range plans involved the Senate or the White House. He said, "In my family we have learned not to make long-range plans." Pretty chilling answer.

So, politics aside, it's warming to hear a long 27 years later that he is musing on, if not actually planning, his re-election campaigns in 2006, 2012 and 2018.

Theo Lippman Jr. is a retired editorial writer for The Sun and author of Senator Ted Kennedy: The Career Behind the Image.