Japanese real estate investors buy but they never, never sell. That has been the conventional wisdom as these well-financed companies have spent tens of billions of dollars on American hotels, office towers, condominiums and golf courses in recent years.

Now the buying is slowing as U.S. property markets sour and changes in the Japanese economy put new pressure on the investors. In the process, the conventional wisdom that what a Japanese buys he keeps is turning out to be not quite right.

Shuwa Investment Corp., holder of 40 U.S. properties worth more than $2.8 billion, said last week that it does not rule out sales. "Shuwa may from time to time sell a property if the capital involved can be better utilized for new acquisitions," the company said in a statement.

Kumagai Gumi Co., a construction and real estate firm, said last week that it has sold a few condominiums and would like to unload some U.S. office space. But for now that hasn't been possible, because the U.S. market is "very soft."

None of this is to say that Japanese investors are pulling out of the United States. "Under no circumstances ... will Shuwa sell any of its most important flagship holdings," the company statement said. Among these are the Arco Plaza in Los Angeles and the ITT Corp. building in New York.

And new investment dollars continue to flow east across the Pacific, but at a slower pace. Kenneth Leventhal & Co., a Los Angeles real estate accounting firm, estimates that Japanese purchases will total $10 billion to $13 billion in 1990, down from the $14.8 billion of 1989. Other analysts, however, feel the slowdown this year is much greater than that.

Shuwa alone has made major purchases in San Francisco, Philadelphia and Los Angeles in recent months and declares itself fully committed to the U.S. market. And the owner of the Heavenly Valley Ski Resort near Lake Tahoe in California announced yesterday the property would be sold to a Japanese firm, Kamori Kanko Co.

Still, the old enthusiasm for the U.S. market is not there. An executive from Mitsui Fudosan, whose $1.5 billion in U.S. holdings includes the former Exxon Corp. building in New York, said his group has no need to sell. "Instead of that, we should constantly watch what's going on" in the months ahead, he said, "and during that period stay away from any new investment."

Most analysts believe that changes in the Japanese economy are the main causes of the slowdown in American deals, not the downturn here. Key is the rise in Japanese interest rates to levels comparable to those in the United States, which has wiped out much of the advantage that was formerly to be gained investing here with money borrowed in Japan.

Masahiro Kawakami, senior vice president of Mitsui Real Estate Sales New York Co., a Japanese brokerage and syndication firm, notes that in the old days "people would borrow money from the bank with 4 to 6 percent interest and buy property which created an 8 percent return. So they could pay all the interest using the return from the investment." With Japanese interest rates now hovering in the 9 percent range, that doesn't work anymore.

The Tokyo stock market's 37 percent fall since the start of the year has also caused companies to pull back. Much of their U.S. expansion has been financed by borrowing against those stock holdings. At the same time, due to market conditions and the prodding of bureaucrats in Tokyo, Japanese banks have become less willing to make real estate loans, whether foreign or domestic.

The softening of stratospheric land prices in Japan itself has also had an impact. "Many investors had borrowed against real estate in the past to make investments in the U.S.," said Robert D. Lynch, a vice president at Nikko Securities Co. International. "And with the price falling their borrowing capacity has diminished."

Politics is also acting as a brake. The Japanese Ministry of Finance is widely reported to have twisted the arms of insurance companies to go easy on big-ticket real estate deals in the United States to minimize simmering anger over national icons being "bought up" by foreigners.

From the start, Japanese investors in the United States have been faulted for operating with a naivete bred in a home market that has never seen a major downturn since World War II. They spent lavishly on "trophy" buildings such as Rockefeller Center in New York and hotels on Waikiki Beach in Hawaii in the belief that values could only go up. Now some are finding out the hard way that that isn't always true.

"What they've discovered is that, unlike Japan, tenants move out at the end of their leases, that there are often vacancies," said Lawrence S. Bacow, director of the Massachusetts Institute of Technology's Center for Real Estate Development. Fear of war in the Persian Gulf and a full-blown U.S. recession is making Japanese investors even more jittery.

But U.S. real estate prices are low and finding a buyer for them at a price that does not mean huge losses is hard.

"Some properties are going to be sold off," predicted Barbara Pfeiffer, research editor for Japan M&A Reporter, a trade journal that tracks Japanese investment here. "In general, though, the Japanese tend to have a wait-and-see attitude. ... You're not going to see a rush to the market." Correspondent Paul Blustein in Tokyo contributed to this report.