THE DEFENSE appropriations voted by the lame-duck Congress will severely restrict Congress' ability to change its mind about force structures in the future. People at the Pentagon know that well. Do members of Congress?

Congressional unease about uncontrollable future growth in the defense budget has been mounting. Yet both the House and Senate versions of the continuing resolution -- which will control defense spending for at least the next several months -- include, with the notable and perhaps temporary exclusion of the MX missile, all the many weapon starts and continuations sought by the administration. If people in Congress think they can easily reconsider the wisdom of those purchases at a later date, they should think again.

Each new weapon start -- no matter how small the initial down payment -- attracts a powerful constituency in Congress ready to oppose any curtailment on grounds that money will be wasted and jobs lost. As the production lines get rolling, additional claims on future budgets will be made not only to pay for the weapons but also for the often equally huge -- and usually underestimated -- costs of equipping, maintaining and manning them.

Budget pressures in future years are likely to be so great that Congress will have to take actions that are harmful to national security. It could stretch out or terminate new weapons production. More likely it would ignore new developments in weaponry, skimp on essential maintenance and personnel or prematurely phase out still usable ships, planes and other weapons.

The only way to avoid this start-and-stop type of buildup is to proceed carefully, leaving some room in future budgets to adjust to changing circumstances. That means getting specific about where each major weapons system fits into urgent defense needs. What gap, for example, do the two additional nuclear carrier task forces fill? Will the planes they carry have the range and flexibility needed for likely missions? Could the $30 billion or so needed to build and support them be spent better on other unmet needs, such as sealift and airlift? The Army is rapidly changing its doctrine for the defense of Europe. What does that imply about future needs for various ground weapons and air support?

These sorts of questions are being asked by many supporters of a strong national defense. They question whether the administration's budget plans are not too heavily conditioned by the historic buying preferences of the military services -- the Navy favoring big surface ships, the Air Force its bombers and the Army its tanks -- and too little attuned to how rapidly changing weapons technology is forcing changes in military strategy.

Congress has now missed its chance to force a more reasoned distribution of funds, or at least a clearer justification of the administration's defense investments. That makes the discussions of next year's budget, now going on within the administration, all the more crucial. The MX missile debate is important, but it is essentially a debate about strategic doctrine, not about budgets. The administration should also take time to consider how its current spending decisions may limit its future choices with respect to conventional forces.