The gross national product grew at a 2 1/2 percent annual rate in the first quarter, after seasonal and inflation adjustments, somewhat less than reported earlier, the Commerce Department said yesterday.

However, economists said the changes in the economy reflected in the revisions set the stage for faster growth in the current quarter and later in the year.

Using less complete data, the department's Bureau of Economic Analysis had estimated last month that real GNP rose at a 3.1 percent rate during the first three months of the year. Real GNP fell at a 1.1 percent rate as the recession ended in the fourth quarter of 1982.

The department also reported that corporate profits from current production rose at a $20.8 billion rate in the first quarter--a 12.6 percent increase--to a level of $185.4 billion. In the fourth quarter, such profits fell $1.6 billion.

Most of the downward revision in the output of goods and services in the first quarter was due to new estimates of the change in business inventories, which declined at a $37.3 billion rate. That drop was originally put at a $28.5 billion rate.

Estimates for consumer spending, federal government spending and business investment were revised upward slightly. The figures for residential construction, net exports and state and local government spending were lowered.

"GNP was revised downward because the first quarter was a transition period from recession to recovery," said Jerry Jasinowski, chief economist for the National Association of Manufacturers. "A modest recovery got under way early in areas like housing and construction. But because businessmen were not convinced that the recovery was real, they continued to reduce their inventories.

"With inventories now quite low and the widespread pickup in economic activity in April, I expect second-quarter GNP to rise at a 5 to 6 percent rate," Jasinowski declared.

Walter M. Cadette, a vice president and economist at Morgan Guaranty Trust Co., told financial analysts, "The stirrings in the U.S. economy are livelier than they look on the surface." He put the "underlying" rate of U.S. economic growth--which he calculated by excluding farm production and earnings on U.S.-owned investment abroad--at nearly 6 percent in the first quarter.

In spite of the substantial quarterly increase in profits from current production, before-tax book profits rose only 1.3 percent and after-tax profits fell 4 1/2 percent to a seasonally adjusted annual rate of $112.5 billion. After-tax profits fell because profits on inventories all but disappeared in the quarter as prices for many products stabilized, and depreciation charges increased substantially.