WARSAW -- At harvest time three years ago, the Soviet Union's leading agricultural expert went to Hungary and spent two days touring the vast cornfields and poultry houses of Babolna, the country's premier cooperative farm.

The visitor was Mikhail Gorbachev, then a secretary for agriculture on the Central Committee of the Soviet Communist Party, and he must have been an intimidating figure for his hosts. Hungary, after all, was engaged in a daring and lonely effort to reform the socialist economic system by introducing some principles of capitalism. And Gorbachev was a representative of a Kremlin leadership that, as the overseer of the communist system, looked with a mixture of skepticism and distaste on its allies' deviation from orthodoxy.

As it turned out, Babolna's directors had little to fear. Far from preaching, this expert from Moscow was intent on studying. The visiting Gorbachev "asked a lot of questions" and "absorbed everything," Babolna director Robert Burgert recently recalled.

"I am quite pleased that some of the ideas" for subsequent Soviet farm policies "seem to have originated in Hungary," Burgert added.

In fact, 2 1/2 years after Gorbachev moved from his secretary's post to the leadership of Soviet communism, that visit to the model Hungarian cooperative might be seen as a foretaste of the profound shake-up he has initiated in Moscow's economic relations with East Europe.

Since taking power, Gorbachev has turned the Soviet Union from the East Bloc's ideological policeman to one of its most avid proponents of radical economic reform. He has elevated Hungary from the status of maverick to that of model among East European states. He has launched a reorganization of trade and business relations among communist-ruled countries, with far-reaching implications for the future.

At the heart of that dramatic shift, moreover, is a change of attitude as simple as it is sweeping: Gorbachev's Soviet Communist leadership, no longer believing that it knows the right economic recipe for communism, has decided to imitate, rather than dictate to, its more progressive allies.

"No party has a monopoly on truth," Gorbachev declared in a landmark speech in Prague earlier this year. "Some problems that are now priorities in the Soviet Union have already been solved in other socialist countries, or they are solving them in their own way."

In the short term, the new Soviet policies have permitted unprecedented freedom and diversity in the economic practices of states long known for their lock-step imitation of Moscow.

In recent months, Hungary and Poland have prepared major programs to decentralize control over economic activity and encourage private enterprise going well beyond the initial Soviet measures. At the same time, East Germany and Romania have felt free to reject any change in their highly centralized systems despite the Kremlin's example.

Whether such diversity will continue for long is questionable. In addition to promoting internal reforms, Gorbachev has made closer economic integration of the Soviet Union and Eastern Europe an important priority. Changes introduced since 1985 in the trading and investment system of Comecon, as the East Bloc's economic community is known, have increased the pressure on East European countries to move toward internal reforms and to deliver more and better goods to the Soviet economy.

Now, with those partial measures producing meager results, the Soviets have moved to embrace a more radical shake-up of Comecon that, by introducing elements of a free trade system, could make the move of communist states away from centralist socialism both universal and irrevocable.

"The introduction of market mechanisms in intra-Comecon trade will amount to a revolution," said Romanian editor Silviu Brucan in a recent article in Worldpaper. "Conservative leaders clinging to economic orthodoxy face a formidable challenge that is bound to accelerate the changing of the guard."

One of the first steps toward that future is expected to be taken at a meeting of Comecon prime ministers beginning Tuesday in Moscow.

According to official sources, the ministers are due to adopt a long-range plan for reform of the Comecon system that includes the creation in 1991 of the Soviet Bloc's first convertible currency -- the foundation of free trade.

Compared with the ambitious goals of reformers, the present Comecon plan is a modest one, and sources say Soviet officials and their allies in Hungary and Poland

Jackson Diehl, The Washington Post's correspondent for Eastern Europe based in Warsaw, spent a month recently in Moscow. During that time, Gary Lee, the Post's Moscow bureau chief, traveled through Poland, Romania, Yugoslavia and Hungary and earlier had visited East Germany. Their reporting, based on this exchange of assignments, produced a two-part series on the changing relationship between the Soviet Union and its East European allies.

have faced strong opposition from East Germany, Romania and Cuba in promoting the changes. Nevertheless, a Hungarian expert said, "We now have a first step down a path that is clear. Now Gorbachev can use Comecon to force reform."

Soviet and East European leaders have strong incentives to reshape economic bonds. The present trading system, introduced in the 1940s by Joseph Stalin and virtually unchanged since then, is so inadequate that it tends to stifle exchanges among East European countries as well as with the West, slowing the introduction of new technologies and holding down living standards.

The Soviet Union and its six East European allies hold 10 percent of the world's population and some of its most developed heavy industries. Yet total Comecon trade was only $375 billion last year, or less than one-seventh of that of the industrialized West. Trade among Comecon countries currently amounts to little more than $100 billion annually, or about one-third of the total trade of Japan.

Moreover, the goods exchanged between the Soviet Union and its East European allies tend to be the poorest that these countries produce, as each economy saves its best output for export to the West.

Many factories in Eastern Europe have separate production runs for East and West, with the East's run typically receiving cheaper materials, technology and packaging.

It is that reality that largely motivates Gorbachev. "Socialist countries should cease to exchange shoddy goods among themselves and to consider {Comecon} a dustbin," he said in a visit to Bucharest this year.

For the Eastern Europeans, the Stalinist organization of Comecon represents one of the chief instruments of Soviet domination of their region.

Under the present system, East Bloc states are forced by their lack of convertible currencies to depend almost exclusively on the Soviet Union for supplies of energy and raw materials and are blocked from significant economic integration with each other or with Western Europe.

To obtain strategic supplies, the East Bloc states are obliged to ship to the Soviet Union up to three-quarters of the machines and 50 percent of the total goods that they export. They also must invest vast resources in huge, often wasteful, energy development projects inside the Soviet Union.

This trade, in turn, has little to do with prices or money, as socialist countries cannot easily determine the real value of their products or readily exchange their currencies. Instead, each year trade ministries in each country meet with Soviet officials and one another to draw up detailed agreements specifying which and how many goods will be exchanged, from steel and oil to matches and perfume.

For every country, the goal in annual negotiations is to export as little as possible -- and to avoid earning any surplus of money.

"Nobody's out to make a couple of rubles," explained Adam Barszczewski, an official of the Polish Foreign Trade Ministry. "Because you can't do anything with a ruble unless it's backed by goods. What we have is an exchange of goods, not money."

However primitive, this barter system meshed with and reinforced the centralized administrative system of central planning developed by Stalin to run the internal economies of the Soviet Union and Eastern Europe. Consequently, as long as the socialist countries clung to the model of central planning, the Comecon system could not be changed significantly.

What has decisively shifted the balance for Comecon has been Gorbachev's move in the past year to adopt a Hungarian-style market reform in the Soviet Union. The change means a drastic reduction in the role of central planning and independence for factories to seek profits on the marketplace. Like the Hungarians and later the Poles, the Soviets now need a decentralized, market-based trade to complement the planned new shape of the economy effectively.

Moreover, Soviet authorities clearly appear to have been frustrated by the failure of moderate reform measures to improve Comecon's performance. In his first year in power, Gorbachev pressed Comecon to adopt a program for joint ventures among East Bloc firms as well as a plan for enterprises in different countries to negotiate trade and investment agreements directly, instead of going through foreign trade ministries.

The measures seemed designed to circumvent the unwieldy system of annual trade agreements without destroying it. But Soviet and East European officials concede that the programs have failed to produce significant results. Only about 500 of the Soviet Union's 36,000 enterprises have established direct links with East European firms, and only seven have created joint ventures.

"The mechanism is very weak, beginning with finding partners," said Ruben Yevstigneyev, an economist at the Moscow-based Institute of World Social Systems. "All of these cooperation initiatives should be more flexible, but they're not. We don't have the economic conditions for putting them into effect."

The new Comecon plan, which Gorbachev proposed to East Bloc leaders at a summit meeting in November, is intended to begin the process of reorganizing the basic system.

The central element, the creation of a convertible currency for use within Comecon, implies a fundamental change in the operation of Comecon trade as well as in the management of each of the East Bloc countries.

With a convertible currency, East European countries would be free to trade among themselves without being limited by the need to balance each product sold with an equal and opposite product. East Bloc factories producing the same goods would be forced to be competitive to make sales within Comecon.

If fully implemented, the reform would give East European states a real incentive to trade with one another and not just with the Soviet Union. And each country would have to adopt a realistic price system influenced by the world market. Huge subsidies hidden in the present system -- mostly from the Soviet Union to Eastern Europe -- would be eliminated.

The implications of the change to convertible currencies and free trade, even when limited within Comecon, are so great, and the process of implementing it so complex, that Comecon officials finally agreed on only a partial and gradual introduction of convertibility.

According to the plan due to be adopted this week, the system will be phased in over a period of 10 years beginning in 1991, sources said.

The reform will begin only with specified classes of products and include only those Comecon countries that volunteer to participate. So far, sources said, only Poland, the Soviet Union, Hungary and Bulgaria have agreed.

Just as the centralized Comecon system has been a barricade for previous East European reform efforts, the reorganization of the system according to market methods will place strong pressure on conservative East European governments to abandon their orthodox practices, experts say.

"In the past, the differences that existed between the economic systems in socialist countries didn't play an important role for pacts between countries," said Lev Klepatski, a Soviet Foreign Ministry official charged with Comecon affairs. "Now, with the reforms of the system, it will be very important to have coinciding economies in details: systems of prices, systems of wages, currency values."

Yet even if hard-line orthodox states such as East Germany and Romania are dragged toward market-based economies by the Comecon plan, a fundamental change still might be found in the means of force.

For once, East European experts say, it should be objective economic forces -- such as market opportunities -- that prompt shifts in policy, not Moscow's political dictates.

Moreover, the system toward which the Soviets, Hungarians and Poles are guiding the East Bloc is one in which East European countries should have significantly more economic and political autonomy.

In the past, the absence of meaningful prices, market-based trade, or decentralized management meant that East Bloc economic exchanges were determined essentially by political decisions. Inevitably, the strongest partner -- the Soviet Union -- was often able to force its arbitrary terms on East European states.

In the future, that overweening political element should be increasingly curtailed if the East Bloc succeeds in the aim of creating its own market forces, providing objective criteria of price and profitability on which economic exchanges can be judged.

If a huge investment in a Soviet energy project is not cost-effective -- a judgment now impossible to make in a moneyless system -- it will be easier for East European budget managers to say no to Moscow.

In a larger sense, Gorbachev's abandonment of orthodox Soviet central planning for the market methods has also buried the notion that there can be a "scientific model" of East Bloc socialism determined by the Soviet party.

Instead, East European states are being encouraged to conduct their own search for economic practices that work.

In fact, Gorbachev seems to have committed Moscow to an open-ended process of searching for a workable form of socialism together with its East Bloc allies. And it is perhaps a sign of the task ahead that officials on both sides concede that they are not sure where the search for solutions will end.