A relaxed, jovial Leonid Brezhnev told U.S. officials and businessmen at a Kremlin dinner last week that the Soviet Union is now preparing its next five-year plan of national economic development for 1981-85 and can see a big place in it for the Americans.

It was the kind of statement that any businessman looking for new markets and new profits would love to hear. But was it anything more than polite talk from an expansive host?

The Soviets have submitted to the Americans a list of 28 industrial projects ranging in size from a $7.5 million factory in the Moscow suburbs to produce Levi Strauss blue jeans, to a mammoth stee rolling complex costing up to $400 million with Armco Steel Corp. as the prime contractor.

The unofficial U.S.-U.S.S.R. Trade and Economic Council, a bilateral group seeking to improve Soviet-American commerce estimated that if all the projects were approved, it could mean an additional $10 billion minimum of increased trade for the United States and bring thousands of new jobs to U.S. shores.

But well-informed soorces in the U.S. delegation headed by Treasury Secretary Michael Blumenthal that talked for two days with the Soviets, described the list as "something of a test" of Amercian intentions. Vladimir Sushkov, deputy foreign trade ministe, remarked at a press confence that the list was a "preliminary one, to demonstrate the desire of the administration to meet us halfway."

Three projects on the list already have been approved by the United States. including a manufacturing facility for tires for earth-moving equipment, valued at a total of $80 million of which U.S. companies will share a sizable portion.

The other projects provide a king of rough indication of what the Soviets think American industry can offer them and what they need to continue modernizing an economy whose growth rates have fallen consistently through the 1970s.

Some will be relatively easy to agree on, such as assistance to help improve Soviet tobacco production. Others are probably years away, due to Soviet hard currency shortages and shifting national priorities or American concerns over technology transfer that could eventually aid Soviet defense industries or other wise harm U.S. national interests.

Those involving Soviet economic priorities are understood to be a proposal to modernize the Moskvich auto plant with help from General Motors to produce up to 250,000 new model passenger cars a year, replacing the outmoded Moskvich. The Soviet auto industry, now making about a million cars a year, principally at a gigantic Fiat plant, is not earmarked for quick medernization.

Among the projects sure to have political implications in Washington are several for speeding oil and gas production. Although the Carter administration takes the position that it supports increased oil exploitation, some in Congress oppose American aid to expand Soviet oil and gas production on a variety of grounds.

The Trade and Economic Council asserts that all these projects, if approved and under the very best financing conditions, could mean up to $15- $20 billion in new Societ-American trade volume, bringing thousands of new jobs to the United States. Others peg the meaningful total at perhapes $10 billion over the next few years, although all the Amercian sources quickly cautioned that prediction in matters of Soviet trade is a difficult art.

A case in point is that of Soviet meat purchases from Australia. The Soviets in recent years built up a steady trade in beef from Australia and mutton and lamb from New Zealand. In 1977, this beef purchasing program alone stood at about $150 million. The frozen, top-quality beef, shipped to Baltie and Black Sea ports, was used to produce vast quantities of sausage by "stretching" the beef with cereal, fat and other ingredients to help satisfy the demands of this meat-short nation.

This year, the Soviet without explanation have cut their beef purchases close to zero.

"They've put the hard currency elsewhere for this year," said one source.

The Soviets have established ambitious goals for increased workplace efficiency and productivity in 1979. Few analysts here see how they can achieve these goals and if they fall short, it will be impossible to fulfill the economic expansion outlined in the current 1976-80 five-year plan. Soviet heavy industry is planned to rise 5.8 percent compared with a 4.7 percent this year and light industry consumer goods are to rise 4.6 percent compared with 3.7 percent for 1978.

Increased labor productivity must achieve a 4.7 percent rise to reach economic expansion goals, but this crucial category of economic efficiency fell below even the 3.6 percent goal set for 1978.

Each year during this five-year plan, the Soviets have fallen slighty behind one or more of their major glals, so that it is now virtually impossible for them to make up all the ground lost and come out in 1980 even with the targets. Brezhnev criticized sloppy planning and production two weeks ago, but the Soviets know that to make things more efficient in the short run, they need Western technology.

Present Soviet external debt stands at $16 billion, which they say is only a iny portion of the entire national product, and is not a factor that concerns them in borrowing to make the modernization purchases they need.

American concerns over the technology transfer issue stand as a major impediment in this area, but the Soviets have increasingly found alternative sources for the computers they want, although they consistently have been interested in advanced American models.

The Soviet attitude toward trade with the United States is an interesting one. Although they want the restrictive Jackson-Vanik amendment repealed, they also assert that U.S.-Soviet trade is so insignificant it does not amount to enough to worry about. In a recent interview, for example, Dzhermen Gvishiani, the son-in-law of Premier Alexei Kosygin, told reporters at his State Committee on Science and Technology that U.S.-Soviet trade was "less than two-hundredths of 1 percent of our gross national product, so it is of little concern, really."

Sources in the Blumenthal delegation said that affer the two days of talks, they had concluded that the Soviets want Jackson-Vanik repealed as much for symbolic as for practical reasons.

Sushkov was asked last week what concrete steps on emigration the Soviets were prepared to take to help improve the future possibility of repeal.

Sushkov said there was "no connection" between emigration and trade and that the amendment had been a "failure."

Later, he said that Jackson-Vanik should be repealed because it makes trade conditions "uncertain."

For the Americans, whose trade with the Soviet Union will top $2 billion this year, Soviet-American commerce has never fulfilled the promise of the early years of detente. That is clear. What is still unclear is how far the Soviets are willing to go -- or can go -- in improving their performance on trade to ensure that there indeed will be an American role in the next five-year plan.