In recent years, 600,000 of the nation's 900,000-plus hospital beds have been filled each day -- an occupancy rate so low that the average hospital has made no money on the care of patients since 1986.

At the same time, the industry has been laying out $16.5 billion a year in capital and interest for buildings and major equipment like Magnetic Resonance Imaging machines (MRIs).

Many critics say the capital expenditures are a major reason why health care costs are rising faster than virtually any other item in the economy.

That conviction has spurred proposals to change laws that encourage hospital capital investments.

The administrator of Medicare, Gail Wilensky, said the problem is that often new investment simply adds facilities and equipment duplicating what is available in a community -- and the government, through Medicare, is paying for a substantial portion of the investment.

Hospitals making such investments hope to capture a larger "market share" by equaling or besting their competitors in a technology race, she said, but if they then fail to win more business the new facilities and high-tech machinery may simply sit idle.

In other cases, capital investment may lead to wasteful additional operating costs, she said. If a hospital buys an expensive new machine like an MRI but fails to get enough patients to use it, it may be tempted to boost revenue by encouraging tests that are not absolutely necessary. And then, other health officials said, a whole additional pack of unnecessary costs for labor and preparation of the patient is added.

Enlarging on this theme in a memorandum sent to Capitol Hill Thursday and obtained by The Washington Post from a member of Congress, Wilensky said:

"Nationwide more than one-third of hospital beds are empty. But Medicare's capital financing system prevents proper incentives from shrinking this excess capacity. Similarly, hospitals have no incentive to limit equipment purchases by sharing with nearby hospitals. Instead, hospitals buy expensive equipment to maintain their competitive edge, while the need for services may not be adequate to fully utilize such equipment."

She called it a "virtual arms race."

The industry, led by the American Hospital Association, which is meeting here today, strongly objects to proposed changes and is expected to undertake a strong lobbying campaign to kill them.

Paul Rettig, executive vice president of the American Hospital Association, charged that the proposals being discussed assume that hospital capital outlays are out of control. But, he said, "Hospital capital investment is not out of control, it has been steady for the past five years at about 9 percent" of the total cost of care.

"A good portion is not going into new beds -- a lot is for renovation and outpatient care," he said.

Rettig noted the government is already limiting its payments for the daily care of Medicare patients, adding that a questionable profit outlook exerts a strong brake on wasteful capital investments. A survey by the Catholic Health Association found that of 620 hospitals in the group, only 18 had capital projects involving new beds. New Reimbursement Plan

The most immediate proposed change alters the way Medicare reimburses hospitals for capital outlays attributable to Medicare's share of the overall hospital patient load. Wilensky will soon announce the details of the plan, which Congress ordered several years ago, and which will automatically be phased in starting Oct. 1 unless Congress alters it or blocks it.

At present, Medicare reimburses hospitals for 85 percent of their actual capital costs attributable to Medicare patients. Critics say this system leads to wasteful investment because it guarantees the hospital virtually unlimited reimbursements.

Under the plan, instead of repaying hospitals for their actual costs on future capital investments, the government would start paying hospitals an annual fixed capital allowance for each Medicare patient, set in advance. If a hospital had no capital investments, it could use the money for other things; if its capital investments exceeded the allowance, there would be no extra payments.

In the year starting Oct. 1, Medicare's total nationwide payments for capital reimbursements under the new plan would be about $7.3 billion, sources said, or about what they would be under the old system. But the total would be split among all hospitals, not just among those actually making capital investments. So a hospital making large capital investments would get a lower payment than if it received 85 percent of actual costs.

In her memorandum, Wilensky said the plan "will reduce the growth in capital spending in the long run {but} it is sufficient to finance real hospital needs."

Wilensky has not disclosed what she will recommend as the capital payment. Industry spokesmen think the average basic capital allowance on future investments will initially be set at between $300 and $500. The payment will be made as an add-on or "fold-in" to the flat-rate payments Medicare uses to pay hospitals for their day-to-day operating costs for Medicare patients.

In 1989, hospital outlays to pay for capital and interest for new plant and equipment came to $16.5 billion, about a third of which was paid for by Medicare, according to the American Hospital Association.

The AHA, American Protestant Health Association, Catholic Health Association and the Federation of American Health Systems oppose the change.

Mike Bromberg, director of the federation, said hospitals fear the new flat-rate capital allowances will not be raised sufficiently each year to cover the cost of new technology and keep pace with inflation.

Rep. Fortney "Pete" Stark (D-Calif.), chairman of the House subcommittee that handles the Medicare hospital program, was the original sponsor of the provision under which Wilensky is developing the new capital plan. He still favors the idea.

A study committee of the Business Roundtable, a group of 200 businesses, has proposed another possible restraint on capital investment, though it has not been endorsed by the Roundtable. The study group recommended ending the right of nonprofit hospitals to raise capital investment funds by selling revenue bonds on which the interest is tax exempt.

The thinking behind the recommendation, said one source, is that "with occupancy rates of 60 percent to 65 percent there is a great deal of unused capacity," and it "probably doesn't make sense any longer" to let hospitals use tax-favored bonds to raise more. A similar proposal was recommended recently by the Health Insurance Asssociation of America.

Nonprofits constitute about 80 percent of all community hospitals and, according to William Cox of the Catholic Health Association, get most of their capital funds from such revenue bonds. Industry Opposes Proposal

Fred Graefe, counsel to the American Protestant Health Association, said, "If adopted, this recommendation would make it extraordinarily difficult for nonprofit hospitals to make required fire and safety code changes, or to purchase the high-technology, cost-effective medical equipment which serves to reduce health-care costs." The likely impact, he argued, would be that nonprofit hospitals would have to turn to the government for their capital needs.

Some members of Congress are toying with variations on the basic proposals. Rep. Brian J. Donnelly (D-Mass.), for example, is looking at a plan to reduce Medicare capital payments to hospitals with low occupancy rates, on the theory that they should not be paid to expand their facilities when they cannot fill the beds they have. And he is considering trying to limit a hospital's tax-exempt debt to $150 million outstanding at any one time if the hospital does not provide substantial charity care.

Stark said he favors curbing tax exemptions enjoyed by nonprofit hospitals that give little charity care. "Charity care is a snare and a delusion" at many hospitals, he said, adding, "In my county, the hospitals provide less in charity care than they save on their exemption from real estate taxes."