ON TENNIS COURTS, at cocktail parties, in board rooms, at home, corporate executives often die suddenly and long before their time. These are talented people that have been nurtured by management like gifted children. Their funerals are long and somber, replacements are hard to find.

Somewhat coldheartedly, industry estimates that premature deaths are costing U.S. companies $19.4 billion a year in lost productivity. It also estimates that employees at all levels annually drain $3 billion while taking sick leave, and tick away the sme amount in time off for hospital stays and doctor visits. Heart attacks alone cost close to 132 million workdays a year.

These statistics help justify an annual outpouring of over $2 billion for corporate physical fitness programs.

Some 50,000 companies, including 300 giants like Mobil and General Foods, offer Programs that often are as scientifically planned and engineered as any marketing strategy. The idea, say corporate cardiologists, exercise physiologists and physical fitness PhDs, is that improved employee health will boost morale, vigor, productivity and longevity, and thereby trim corporate losses.

The concept is not new. In the 1890s the National Cash Register Co. instituted what was probably the first such program. At the time, feisty NCR president John H. Patterson Liked to assemble his employees at dawn for rousing prework horseback rides. Afterward he thought they seemed more alert. Then in 1894 he incorporated his idea into the regular workday hours with morning and afternoon exercise breaks. Ten years later he built an employee gym; then, in 1911, a 325-acre recreation park.

In the 1950s, the statistical significance between physical inactivity and coronary heart disease was first demonstrated. A study showed that London bus conductors had a lower incidence of heart communication than more sedentary bus drivers; London mailmen were found, to have a lower incidence than postal clerks.

Since then more statistics have been stockpiled and more corporations have built gyms, encouraged corporate sports and hired recreation and fitness program directors.

Of all programs, probably the most famous is that of the Indian Packing Co. of Green Bay, wis. The pro football Green Bay Packers began as an employee fitness team.

Rockwill International started one of the most ambitious programs at El Segundo, Cal., in 1960. Its goal is to have every Rockwell employee and his family follow a daily exercise plan.

At Exxon's Manhattan headquarters the program is less inclusive but more intensive. High above Rockefeller Center, 300 executives (mean age: 46) spend an hour three times a week in the sunny fitness lab progressing through 10 exercise stations - wall-pulley weights, dumbells and punching bags, for example-under the supervision of white-coated Exxon medical staff.

Locally, most impressive is the program at Xerox in Leesburg, Va.

Xerox has stressed employee fitness since 1965. In 1974, when the multinational corporation built its training school with "living-and-learning" accommodations for over 1,000 salespeople service technicians, and mid-level managers at Leesburg, it also erected a mammoth fitness center.

Surrounded by over 2,000 acres of woods and rolling hills, the structure has a sand-colored block exterior, fired clay roof and interior African mahogany siding reminiscent of Wolf Trap Farm Park.

In and around the center, which is open to every Xerox employee, are four basketball courts, seven volleyball courts, six badminton courts, four tennis courts, four paddle tennis courts, four horseshoe areas, two squash and two handball courts, four handball/squash/tennis practice walls, a one-mile jogging trail, an indoor jogging track, a softball diamond, a football/soccer field, swimming pool, 18-hole putting green course and an exercise room with mechanical treadmills, stationary bicycles and a 16-station circuit trainer.

To say nothing of works in progress - a nature trail, for example - and lockers than smell as sweet as newborn babies.

Facilities alone, however, do not a fit employee make, according to W. Brent Arnold, 33-year old manager of physical fitness and recreation for Xerox.

Arnold and his staff, like many other corporate fitness leaders, offer stress testing conducted on inclined treadmills. Using electrodes and other equipment hooked to the body it measures heart rate, tension, blood pressure and muscle flexibility. It can detect cardiovascular complications; in fact, it is believed to pinpoint 70 per cent more abnormalities than a resting EKG (electrocardiogram), Arnold said.

"And even though our students here might be younger than your average executive, heart disease can strike any time," Arnold added. "I had a 22-year old D.O.A. here last year." That was former University of Maryland basketball star and aspring Xerox salesman Owen Brown, who collapsed on Feb. 5, 1976.

Stress test results also help determine an individual's exercise prescription, they set levels for exercise intensity, duration and frequency. Experts say it's the only way to total fitness, and are chagrined by doctors who vaguely advise patients to "get some exercise." To them, that is as absurd as "get some medicine."

But while the physiological benefits of exercise may be proven, other effects are still not easily measured. Companies increasingly bank on the idea that healthier employees are better employees, but there are precious few facts to back that up.

"I get five to 10 letters a week from companies who want to start programs," said Arnold, "but all we know are the tangibles, not the intangibles."

To collect the data that may prove more situpe means higher morale means lower costs, the American Association of Fitness Directors in Business and Industry (AAFDBI) was formed in 1974.

Last October, Xerox hosted AAFDBI's second annual conference. For two days over 125 company fitness leaders from the U.S., Canada and Japan discussed topics like the role of exercise, tension and relaxation in decision-making; flexibility and lower back pain; motivation and behavior modification.

Arnold, AAFDBI's new president, says the data will be used to improve existing programs and to help convince skeptical executives to invest in employee health.

Evidently, some executives still oppose the idea of employees exercising on company time; but generally, says program supporters, foes are those personally unaware of the benefits of fitness.

"When some people get the urge to exercise," said Arnold, "they lie down until it passees."