For Defense Secretary Caspar W. Weinberger, life isn't all MX missiles and cream.

Weinberger is a fan of skimmed milk. And, according to internal -- but unclassified -- documents obtained by The Washington Post, that preference, along with Weinberger's ceaseless quest to reduce his $314 billion budget without endangering national security, eventually may affect the diets and waistlines of military personnel all over the world.

The documents show that an aide to the chief of staff of the top-secret National Security Agency, Dennis C. Chadwick, suggested in 1983 that the military could save $4 million per year and improve soldiers' health by serving them low-fat (2 percent) milk instead of whole milk in its dining facilities.

The documents also show that Chadwick met with rebuff after rebuff in his efforts to turn suggestion into policy. They show that last month Chadwick, out of frustration, sent Deputy Defense Secretary William Howard Taft IV a memo that, somehow, wound up on Weinberger's desk.

"Will, what's this -- " Weinberger scrawled on the memo. "Watering the milk will save $4 million?"

That was Jan. 8. After six days of contemplation or, possibly, research, Taft sent a memorandum to Weinberger, dismissing Chadwick's idea and, in the process, apparently confusing 2 percent milk with skimmed milk.

"CWW:" Taft began collegially. "The proposal rests on the proposition, true as far as I know, that skim milk is cheaper and better for you than whole milk. It ignores the proposition, also true as far as I know, that servicemen don't like skim milk, even if they haven't tried it.

"On balance," Taft concluded, "it seems to me better to stay with whole milk. WHT IV."

Weinberger pondered this response for a day, the documents show, and then last Tuesday reluctantly accepted his deputy's advice. Underlining Taft's assertion that "servicemen don't like skim milk," Weinberger penned, "OK."

But the secretary could not quite let the matter rest.

"But we might try a controlled experiment -- a blind test so to speak -- to see if they really don't like skim milk," he added. "I do!"

Weinberger underlined his "I do!" and added a note of encouragement, and caution: "Mr. Chadwick should not feel that his efforts are ignored, nor should he become too obsessive about it."