In midtown Manhatten, General Motors Corp. announces it's trying to sell its New York headquarters building.

From St. Louis, May Department Stores Co. sells half interests in some of its shopping centers.

And, in Los Angeles, Getty Oil Co. puts its Wilshire Boulevard world headquarters building on the block.

From coast to coast, major U.S. corporations increasingly have been posting "For Sale" signs on some of their choicest commercial properties.

Although statistics are not kept charting the volume of commercial real estate sales in the country, experts across the nation say they see a trend with no end in sight.

"Commercial real estate obviously is the hottest market around right now," said Kenneth R. Anderson, research manager for the Institute of Real Estate Management in Chicago.

The reasons that corporations are putting their primie properties up for sale are many.

Ailing firms are scrambling for money wherever they can find it. Healthy ones often prefer to finance their growth by selling assets rather than borrowing funds at 20 percent.

Some companies are looking to take undervalued real estate off their books in order to boost sagging earnings. Others simply want to rid themselves of surplus property.

But, while the ranks of sellers apparently are multiplying, real estate observers say there still is not enough commercial real estate on the market to meet investor demands.

"There will be more money than there are properties through the near future," Anderson said.

The action particularly seems hot in the office building market. Prices can be staggering -- such s the $400 million that Metropolitan Life Insurance Co. paid for the Pan Am building in New York last year. GM reportedly wants $500 million for its New York building.

Even so, notes Howard Sadowsky, senior vice president of the Julien J. Studley Inc. real estate sales and consulting firm, "more companies are trying to own buildings than ever before."

The motivations behind companies moving in or out of commercial real estate investments are individual, but the cases of GM, May Department Stores and Getty Oil provide a representative sampling.

The General Motors announcement came against a background of record losses for the entire U.S. autombile industry and at a time when the company is trying to modernize its manufacturing operations but faces exorbitant interest rates on borrowed funds.

Ford Motor Co. and Chrysler Corp., confronted with the same set of circumstances, also have sold commercial holdings in recent months.

"For the automotive companies, it's a combination of high interest rates and an absolute need for money," said Steven Miller, senior vice president at Oppenheimer Properties Inc., New York, a subsidy of Oppenheimer & Co.

Pan American World Airways likewise was operating in them midst of industrywide malaise when it sold its 59-story office building last year.

But May Department Stores, owner of the May Co. chain and others, set out to sell a half interest in several of its shopping centers at a time when sales and profits were growing and further expansion was planned.

Lynn Thurber, vice president of Morgan Stanley Realty in New York, said the May move is typical of a growing number of healthy companies that would rather sell valuable real estate holdings than borrow capital at current rates.

"These companies have a requirement for funds and they see that raising money from traditional sources is very expensive," said the executive at Morgan Stanley, which handled the May Department Store transactions.

"They're sitting on a valuable asset, and now is a good time to sell. It's a case of looking to the future, seeing a need for funds finding an attractive way to finance at a low interest rate."

Selling valuable real estate assets also is being viewed as a way to boost earnings at a time when a generally sluggish economy has cut into profits. (Real estate is shown on the corporate books at its original value minus depreciation, regardless of its current market value.)

"Properties on the corporate balance sheet at book value generally can be sold at many times that," said Claude Ballard, senior vice president of the Prudential Insurance Co. of America. "They're converting an asset to something Wall Street recognizes."

Still other firms, like Getty Oil, simply find themselves with surplus property on their hands and no desire to keep it as an investment.

"We did not consider keeping the buiding," Getty Vice President Stuart Evey said of the company's 22-story building here. "We're not in the real estate business."

Standing at the head of the line to buy up the investment-grade properties other companies don't want are insurance companies and their client pension funds.

With low-interest, long-term commercial mortgages just about as plentiful as low-interest, long-term residential loans -- which is to say, almost nonexistent -- insurance companies and pension funds are among the few potential buyers with ready access to capital.

Additionally, the pension funds are operating under the mandates of the Employment Retirement Income Security Act of 1974, known as ERISA, which in part ordered pension funds to diversify their investments.

"In Europe, its common for pension funds to have 30 percent of their assets in real estate," said C. Wesley Poulson, president of Coldwell Banker in Los Angeles. "In this country, it's closer to 1 to 2 percent. Our surveys indicate that figure will grow to 12 to 15 percent in the next few years. With $600 billion in pension funds and endowments in the United States, that means an estimated $72 billion will come into equity markets."