WHEN Dwight D. Eisenhower arrived at the Pentagon in 1946 he did not like what he saw. The array of paunchy officers miserably out of shape, their condition exaggerated by the short Ike jackets then in vogue, distressed the man whose life's work had been preparing armies for the great physical rigors and discomforts of battle.

He soon learned what others before had come to accept as a military fact of life: In this enormous facility, intended to house more than 34,000 workers including military officers duty bound to keep in good physical condition, there was no athletic facility for keeping in shape or just doing a few conditioning exercises to relieve the day's tensions.

Officers detailed to the Pentagon from field assignments where it was part of the daily routine to keep in shape found that the physical requirements of Pentagon job were no more demanding than commanding a desk or piloting a swivel chair.

The building's 17 miles of corridors offered little help for the legs because the engineers had designed the 35-acre structure so that the maximum walking distance between any two offices, once one learned his way, was only 1,800 feet, a six-minute walk.

Any of the building's five concentric rings would have made a good indoor track. The outside ring is a mile around, but jogging in the halls would have shattered the decorum of this cold, austere edifice.

It was under these conditions that Eisenhower decreed that there would be an athletic center in Pentagon, and while he did not say precisely where it should be and what it should contain, he made it plain that no officer, thereafter, could use his sedentary Washington assignment as an excuse for being out of shape.

What grew out of Eisenhower's decree is now the busiest gymnasium in the metropolitan area. Buried 50 feet under tons of dirt and concrete, in what was intended to be a bomb shelter but was in use as a bulk storage area, is the Pentagon Athletic Center.

It has some 5,000 dues-paying members and a daily average of 1,300 men and women use it. No longer an exclusively male officer stronghold, the center numbers among its members 1,000 civilians, 375 women and 155 enlisted men.

The current good news for Army Maj. Marion M. Robinson, who runs the facility, is that his request to install women's lockers and showers has been approved. The lack of such facilities for women has presented the center with some moments and has had the effect of denying women equal access.

The center is open daily - except Christmas and New Year's - from 7 a.m. to 10 p.m. Many members begin or end their Pentagon day with a work out. And there are others who run between home and work, pausing at the center to change clothes.

The number of joggers who spill out of the center to run one of the four courses along the Potomac is estimated at about 1,000, but not all on a daily basis. There are many who do run daily, and there are those interate joggers who, running three or more miles daily, had 1,000-mile years in 1976.

Excluding the hundreds of runners who do their outdoors, as many as 200 people can simultaneously use the facilities and from 11:30 a.m. to 2 p.m., when there is the greatest convergence of members, the place is bedlam. The cacophony of hard rubber balls ricocheting off scarred cement walls, the staccato thumping of punching bags, the clanking and squeaking of rowing machines and exercycles, the shouts of volleyball and basketball gloves testify to the intense activity.

In this peak time more than 500 persons sweat through workouts of about an hour's duration. Over the 15-hour schedule, the facility accommodates about 800 more, with more leisurely and longer sessions after 6 p.m.

Available at the center are three badminton courts, a golf driving range, seven handball-racquetball courts, eight squash courts, two shuffleboard tables, three tennis tables, a volleyball court, a battery of weight lifting and mechanical conditioning equipment which includes rowing machines, bicycles, chinning bars, punchings bags and barbells. The steam room has individual saunas and standing by is a professional masseur.

There is a three-lane, 75-foot swimming pool which is expected to be the favorite source of exercise for new Secretary of Defense Harold Brown. Former Defense Secretary Donald Rumsfeld was a regular on the squash courts and former astronaut Mike Collins is a frequent handball player there.

The center is so popular - especially for squash, handball and racquetball - that these courts must be reserved.

Busy as it is, and in light of its military origins, it is remarkable that there is no resemblance of regimentation, organized team athletics or group supervision. Each member exercises on his own schedule, does his own thing in his own way. The only conformity required is that each member pay his $84 dues annually and get a physical exam before beginning.

There is a remarkable spontaneity. All games are "pick-up, much as they would be in sandlot basketball," said Robinson. "It's less formal and more fun that way."

Yet out of this informality and unstructured activity - largely self-starting, self-supporting, self-coaching - have emerged some excellent competitive teams enjoying success in local squash, handball and racquetball leagues.

And, when secretaries of Defense, generals, admirals, colonels and sergeants hang their rank on the coat rack outside, all perks disappear under the sweatshirts.