Once before, when he was 48 years old in 1974, economist Alan Greenspan gave up his lucrative practice as economic adviser to some of America's richest corporations to perform the same chore for President Ford. Now, at 61, the erudite, Washington-savvy Greenspan is doing it again, moving this time into the big shoes of Federal Reserve Board chairman Paul A. Volcker.

It is a trade of one establishment conservative for another. But it will take time for financial markets to accept the reality that, after eight years, Volcker will be gone from the Fed. Volcker, to the rest of the world, was a unique symbol of America's financial strength, and security for its investments.

But of all possible successors to the distinguished retiring chairman, Greenspan may come closest to Volcker in his concern about inflation, dedication to reducing the budget deficit, vigorous support for free trade and belief that a sharply declining dollar poses a major threat to the world's economy.

Greenspan is well known in international financial circles though not so experienced as Volcker in negotiating with other central bankers. He is highly regarded by his fellow economists for his competence, forecasting ability and integrity. That includes liberals who ordinarily worry about right-wingers.

Said the late Arthur M. Okun, Lyndon Johnson's chief economist, about Greenspan's appointment to the Council of Economic Advisers: "I sleep better knowing that if the president has to have a conservative economist, at least it's a competent and honest one." Greenspan, a golf-playing companion of Ford's, may have been the most influential CEA chairman in history because of the close working relationship he developed with Ford.

His entree to the White House establishment could give him an influence within the Reagan administration never available to Volcker.

As a disciple of philosopher Ayn Rand's "objectivism," Greenspan supposedly is a laissez-faire economist who believes that the least government is the best government. "The only way to get the economy back on an even keel is to resist the temptation to override free-market processes," he wrote in 1973. "What the economy badly needs is a strong dose of do-nothingism."

But he has let the world, events and his own government experience shape his views into a more pragmatic framework. While Greenspan still thinks a gold standard would be the best basis for the nation's monetary system, he knows it isn't going to happen -- at least in his lifetime.

At a meeting in New York last December on debt and trade issues, Rep. Jack Kemp (R-N.Y.) tried to pressure Greenspan into an endorsement of a gold standard to stabilize the international monetary system.

"I certainly think we need to look at something that will stabilize the system," Greenspan answered. "What concerns me is that the claims in dollars are now so huge that it's going to be very difficult to get a fix on gold, though if we can, I would certainly agree that's the ideal system to reach.

"Even though I would like to see that happen, at the moment it's a type of thing that's going to be very difficult to achieve. Having said that, the alternatives don't strike me at the moment as very propitious either."

Greenspan is also highly regarded on Capitol Hill, where he is credited with the successful management of the commission that "rescued" the Social Security system. He is well liked among journalists for his candor, affability, accessibility and factual comments devoid of a partisan slant.

The sophisticated question his secretary always asks reporters who telephone is: "Are you on deadline?" His good press relations have paid ample dividends. A recent National Journal survey listed him high among the "icons" whose views are sought and quoted on all the major economic issues.

On some of today's critical problems, a Greenspan-led Federal Reserve will be hardly distinguishable from Volcker's. Greenspan has been one of Volcker's biggest fans. And as an "outside" adviser to the Reagan administration, Greenspan consistently fought former White House chief of staff Donald T. Regan's efforts to clip Volcker's power. He sided with former budget director David A. Stockman, and with James A. Baker III (when Baker was chief of staff), in efforts to get Reagan to accept "revenue enhancement" to lower the budget deficit.

But Greenspan has some reservations about the efficacy of international economic-cooperation efforts launched by Baker as Treasury secretary, notably at the famous Plaza Hotel meeting in New York among finance ministers and central bankers in 1985.

Greenspan believes each nation goes to international meetings -- such as the one coming up next week in Venice -- with its own domestic priorities, which leaves little room available for meaningful negotiations. Last July, at a time when the administration would not concede the growing economic threat at home, Greenspan correctly forecast in an interview with The Post that lower interest rates wouldn't help much.

We may be at one of those periods in history where policy has very serious limitations," he said. "You can hope that positive elements emerge. But my concern is that much of what policy can do is already in place."

Greenspan now gets the chance again to review that policy from the standpoint of a key player. The markets will keep score on his efforts.

"His entree to the White House establishment could give him an influence . . . never available to Volcker.