Four House committees will have new leaders this year. In three cases, selections were made to fill vacancies, and the most senior committee member seeking the chairmanship was given the post. Rep. Lee Hamilton moves to head the House Intelligence Committee and Rep. Julian Dixon to the Committee on Standards of Official Conduct. For a while last month, the future leadership of the Budget Committee was uncertain, but by the time Congress convened, Rep. William Gray was the only contender. After his selection he spoke intelligently and graciously to his colleagues, giving assurance that he is both willing and able to deal evenhandedly with all government programs in approaching the budget problems ahead. His task is formidable, and he has taken it on in the right spirit.

The choice of a chairman for the fourth committee, Armed Services, was a different story. The post was not vacant, and the chairman was strongly supported by the House leadership. But a man seventh in line of seniority was chosen to lead the committee. Rep. Melvin Price had been chairman for 10 years. He is 80 years old, has served longer in the House than all but one of his colleagues and is in poor health. The refusal of his fellow Democrats to elect him to a final term as chairman signals more than a judgment on his personal leadership; if that had been the case, they would have turned to the man next in seniority, 74-year-old Rep. Charles Bennett. Instead, by selecting Rep. Les Aspin, they indicated a determination to revise both the image of the committee and its relationship to the Pentagon. This is a healthy sign.

Mr. Aspin is the first Vietnam-era veteran to serve as chairman of the Armed Services Committee. His experience with the military is quite different from that of oter committee members who served during World War II. By education and experience -- he has a PhD in economics and served as a staff member in the Pentagon and on the Council of Economic Advisers -- he is well equipped to lead the committee. A reputation for expertise in Defense Department programs and a conviction that the committee's responsibility is to shape and supervise those programs, not simply to approve them, persuaded fellow Democrats that the Wisconsin representative was the man for this job at this time.

The seniority system works well in many cases and has been particularly useful in assuring minorities the right to leadership posts. But it is not sacrosanct, and it may be on the wane. Five other committee chairmen each received more than a dozen negative votes in bids to retain their posts. Great age and length of service entitle a member to respect but not necessarily to power. Democrats want a new course on the Armed Services Committee, and last week they chose a good man to set the direction.