The Russians are coming, the Russians are coming! As a matter of fact, the Russians are here: Their highest financial and banking officials will be special guests of the World Bank and International Monetary Fund meeting beginning Tuesday.

It's the first Soviet involvement since their appearance at Bretton Woods in 1944 when the two institutions were founded. In the end, they never joined.

Now, they desperately want in. What does it all mean? In principle, the United States and other major powers no longer have any objection to Soviet membership in the IMF and the World Bank.

But first, Moscow will have to prove that the Soviet Union is not exploding into many pieces. It must also demonstrate that it can keep its union of 15 republics together in a drastically reshaped structure that resembles a market economy rather than the "command," state-controlled economy that Soviet President Mikhail Gorbachev has been trying to salvage with perestroika.

Perestroika (literally, "restructuring") assumed that old-style socialism could be given a pretty face. Unfortunately, under perestroika, the Soviet economy deteriorated badly, strengthening the prospect that the union would break apart. Now, as the Soviet economy lies almost in extremis -- on some days, bread is not available, on other days, meat -- the pragmatic Gorbachev agrees: He has moved dramatically toward the 500-day reform plan of economist Stanislav Shatalin that would eventually put as much as 80 percent of the Soviet economy in private hands.

Shatalin would provide for government loans to small shop managers -- the baker, the shoemaker -- to buy such retail and service operations. Industrial enterprises and commercial buildings would be turned into stock companies, with shares sold or given to employees and others.

The "500 Days" strategy calls for creating laws governing private ownership, banking, stock markets, taxation and social security in the first 100 days. The next 150 days would be devoted to privatizing companies and selling housing and land to Soviet citizens (a step in creating the market system that also helps soak up the huge overhand of paper rubles).

About eight months would then have passed. The next 150 days, according to the Shatalin scenario, are to introduce full-market mechanisms, including price decontrol and tough competitive policies. The final 100 days, a year-plus since the start of the program, are called a period of "aggressive stabilization," including public-investment programs, and release to the public of consumer goods and food imports.

Naturally, nothing this complicated is going to fall into place neatly. Most concepts of a market economy are totally unknown in the Soviet Union. And after 70 years of relying on the "command" economy to do their thinking for them, Soviet citizens tend to be suspicious of a society where extra effort gets extra rewards. Most American experts on the Soviet Union doubt that the plan can actually deliver what it promises.

But Shatalin argues that there is no alternative except to try the fast track. He has convinced Russian Republic President Boris Yeltsin and Gorbachev that if his three key steps -- privatization, price decontrol and cuts in government spending -- are put into effect, there will be goods on the shelf that can be bought without ration coupons. An analysis by American Express Bank, the London-based investment subsidiary of the U.S. financial services firm, says that would mean real internal convertibility of the ruble. After that, external convertibility would be a short and logical step.

But Gorbachev is indecisive: He's contemplating a referendum on the emotional and tricky question of private ownership of land, one of the key elements of Shatalin's plan. A referendum, Shatalin says, would delay the whole reform process another six months.

An academic who has moved swiftly to prominence in the last year, Shatalin visualizes a sort of common market among the 15 republics, which would manage their own internal economies, although there would be one common currency. All the republics would sit on the board of a national bank, handling the nation's financial problems. The concept is somewhat like our own "states' rights" view of the Constitution: The Soviet would have all rights and powers not delegated to the union. That may not be acceptable to Gorbachev.

Yet the Russian Republic under Yeltsin seems ready to go ahead with the Shatalin plan, whatever happens, on Oct. 1.

Officials at the IMF, who were commissioned at the July Houston economic summit to come up with recommendations on how the Soviet economy might be helped, welcome the new initiative. For the first time, they see recognition in Moscow of the need for drastic change: Until now, says one official, "it wasn't even possible to engage in a policy debate because there was nothing to bite on."

Robert Hormats, a former top State Department economics official and now a vice president of Goldman Sachs & Co., suggests that the real question is not so much extending membership -- which has symbolic importance to Gorbachev -- but the extent to which the fund and bank can supply expertise and systematic consultation to the Soviet Union.

Hormats, just back from Moscow, said in an interview that the bank and fund could help the Soviet Union set up the new banking and monetary system contemplated by the Shatalin plan. The Soviets also need World Bank support for revamping their agricultural system. This year, one-third of the crop was lost between the farm and the market. There are other basics: A modern telephone system is badly needed. Hormats says it takes three or four hours to make a telephone call out of the Soviet Union -- if you're lucky and if you can speak Russian or German to the Soviet operator.

The American Express Bank analysis lays out the long odds against Shatalin's "radical dash towards a market economy." But it notes that some early success could come from the decision to privatize first, then go to price decontrol. The gamble suggested by Shatalin is that by privatizing first, output and productivity will respond rapidly, so that when prices are decontrolled, a fairly adequate supply will keep prices from going through the roof, as they did in Poland.

As Zurab A. Yakobashvili of the USSR Academy of Sciences observed: "If at the end of 500 days, we have managed to lay just the skeletal foundation of a market economy, I would consider it satisfactory."