MASSACHUSETTS SEN. Edward M. Kennedy was 9 years old when the man he will now succeed as chairman of the Senate Judiciary Committee, James O. Eastland, came to the Senate from Mississippi. That was in 1941. We note this historial fact because it gives you some sense of Mr. Eastland's own political longevity and also of the panorama of social and political change his career has spanned. Mr. Eastland, it needs to be said at once, give over a large part of that career - up until around the mid- to late-1960s - to doing everything in his power to block for retard the principla element of that change: the liberation, first of his region and then of the country as a whole, from the shackles of government-sanctioned, culturally blessed institutional racism.

It does not mitigate the recollection of the especially ugly and raw language in which the Mississippian defended his position in this battle to observe, as many have done, that Mr. Eastland was and remains essentially a politican - and that had he come from Brooklyn, N.Y., instead of Sunflower County, Miss., he would probably have out-liberaled Jacob Javits. Those decades of racial obsession, plus some miserable contributions to the witch-hunting of the 1950s and early 1960s, must stand as central features in any overall portrayal of his career.

Nevertheless, to dwell exclusively on these aspects of Mr. Eastland's 37 years in the U.S. Senate is to miss the meaning of the current transition on the committee he headed and also to miss the contemporary history lesson embodied in the turn his career has taken. Southern senators like Mr. Eastland were themselves unexpected beneficiaries of the battle they lost in Congress and the courts: They were freed up to think about something other than how every act or proposal that came into their line of vision could be used to bolster the doomed racial dispensation of the South. Mr. Eastland in the past decade or so has managed to win - yes - the affection and personal regard of many of those liberals and assorted integrationists who have served on or around the Judiciary Committee. He has, by their accounts, introduced and maintained a high degree of democracy into the committee's proceedings. He has been fair and understanding in his dealings with the members. "He plays hardball with you on the issues," is the way one puts it, "but not tricks." Another liberal admirer elaborates: "He is not like some of those 'high-minded' ones who beat you by calling a vote when you're in the men's room."

That may not sound like much to you, but in Senate terms it means plenty, and it leaves open the question of what will be different, or needs to be different, on the committee under the new leadership of Sen. Kennedy (himself, incidentally, another of Mr. Eastland's personal friends). The first thing to acknowledge is that the Judiciary Committee is no longer a main staging ground for legislation and hearings on racial questions. If there is one large and central concern on the committee that approximates in importance and controversy the steam once generated by racial issues, it is antitrust affairs. Chairman Eastland had been very careful to balance out the antitrust subcommittee in such a way as to keep the lid on. Everything from oil to auto-pricing the health insurance has its antitrust aspect. Conceivably Sen. Kennedy will make the subcommittee a more activist one, just one he is generally expected, in terms of hiring staff and attracting and assigning committee members, to try to give the committee a somewhat more militant, progressive bent. There is also talk of introducing more explicit and objective standards into the process of approving federal judges who have been nominated on the say-so of senators from their states.

Because the investigative power of the committee can be a formidable political tool, because much important legislation must pass through it and because it still has crucial patronage powers and power in private immigration bill passage, there is every reason to suppose Mr. Kennedy can and will turn his new stewardship into a tremendous political asset. It is one of those wholly unexpected little turns of history that one should add that at the same time he will be pressed to uphold a standard of personal fairness established by, of all people, Sen. Eastland.