The Reagan administration predicted yesterday the nation's economy would show absolutely no growth in the current quarter despite unexpected strength earlier in the year.

On a more encouraging note, officials said inflation also has eased, as it tends to do when the economy slows.

The projections came as the Commerce Department was releasing new revisions showing that the gross national product, even after being adjusted for inflation, grew at a robust annual rate of 8.6 percent in the January-to-March quarter.

Shortly after the GNP report was released, Commerce's acting chief economist, William Cox, issued a statement saying real GNP "will show little change" in the current quarter.

Cox said that estimate was "based on fragmentary information" for the quarter, which is now more than two-thirds over, and he declined to release the exact number for what Commerce calls its "flash" GNP estimate.

However, other sources, asking not to be identified, said the estimate shows an absolutely flat zero -- neither growth nor decline. The estimate also shows an annual inflation rate of 6.1 percent in the second quarter, as measured by the broadly based GNP implicit price deflator.

The annual growth rate for "real GNP" -- the inflation-adjusted value of the nation's goods and services -- was revised upward from 8.4 percent and was the biggest quarterly gain since early 1978.

Inflation during the first quarter rose at an annual rate of 9.8 percent, as measured by GNP-connected figures.

In addition to the new GNP figures, Commerce also revised its figures on first-quarter corporate profits yesterday.

Before-tax profits rose 3.8 percent in the January-to-March quarter to a seasonally adjusted annual rate of $259.1 billion rather than rising 3.7 percent as originally reported.

Corporate profits from current production, adjusting for depreciation and inventory replacement costs, rose 10.7 percent rather than 10 1/2 percent, officials said.

The latest revision indicated that real GNP rose to a seasonally adjusted annual rate of $2.853 trillion in the first quarter. It had risen at a rate of 3.8 percent in the final quarter of 1980.

Meanwhile, Commerce officials also said the nation showed a $3.1 billion first-quarter surplus in its foreign-trade balance as measured by the broad current account of goods, services and other financial transactions.

The new report also greatly revised last year's current-account figures, which now show a $3.7 billion surplus in 1980 rather than the $118 million reported in March.

It was the biggest yearly surplus since the $4.4 billion of 1976, but it also was no longer the first surplus since that year, as earlier reported. Commerce now says a $1.4 billion surplus was posted in 1979 rather than the $705 million deficit the department had been citing.

Department officials had indicated earlier that the balance would show a big surplus for the January-to-March quarter, contributing heavily to the unexpectedly rapid 8.6 percent annual rate of growth in the inflation-adjusted gross national product during those three months.

The current-account figures, which also are adjusted for seasonal variations, measure a wide variety of financial transactions, including those involving goods, services, foreign aid, loans to foreigners, the movement of gold and such payments as pensions to Americans who live abroad.

In the first three months of the year, the new report said, "In the merchandise accounts, a large increase in exports, following several quarters of little change, more than offset an increase in imports associated with stepped-up U.S. business activity."