A new round of government statistics yesterday confirmed that the economy has begun to show from its rapid pace earlier this year-a development that may help cool inflation later in the year.

Revised Commerce Department figures showed the "real" gross national product, the measure of the economy's growth rate, rose at a scant annual rate of 0.4 percent last quarter-below the 0.7 percent estimated previously.

The department said inflation, as measured by the comprehensive GNP price index, registered an 8.8 percent annual rate during the period, essentially the same as reported in preliminary figures last month.

At the same time, the agency said profits of American corporations rose far less sharply last quarter than in the final three months of 1978, with the gain coming mainly from inflation and not from increased sales.

The department said before-tax profits of U.S. firms rose a modest 0.9 percent between January and March to a new annual rate of $226.9 billion, compared to a robust 9.5 percent gain in the previous quarter.

After adjustment for the effect of inflation on inventories and depreciation, profit levels declined 6 percent. After-tax profits rose 5.7 percent to a new annual rate of $137.9 billion.

The combination of figures reinforced signs from other economic statistics recently that the economy is slowing visibly-just what the White House says it wants to help slow the inflation rate.

Treasury Secretary W. Michael Blumenthal told the Detroit Economic Club the moderation should "help cool down" the price surge in coming months. He asserted the 13 percent inflation rate so far this year "will not hold up."

Blumenthal also predicted the U.S. trade posture would continue to improve, with the nation's "current account" deficit-trade, services, tourism and earnings on foreign investment-shrinking to a $2 to $6 billion rate by early 1980.

The revised figures on the economy's growth rate dampened speculation that the earlier estimate might have been a fluke. The previous calculation was so low that some analysis speculated it later would be revised upward.

However, late-breaking data showed buying activity was less robust than had been expected and inventories grew less rapidly than anticipated-indicating there was little of the overheating that some policymakers feared.

Analysts said because the first-quarter figures were depressed some by the effects of bad weather in slowing production, they still expect a rebound in the current quarter's growht. But the slowdown clearly has begun.

Corporate profits remained essentially flat during the third quarter of 1978. For the year as a whole, before-tax profits last year rose by 16.2 percent-a relatively moderate gain, following substantial jumps in earlier years.

When the late-1978 profits figures were published, some administration officials suggested the rise was evidence that business was raising prices excessively. These officials, however, have since changed their minds.

The GNP figures yesterday were the latest in a series of statistics recently that indicated the economy is entering a slowdown. Economists are divided over whether a mild recession is in sight.

Earlier this week, the department reported that housing starts declined 2.1 percent in April, while the personal income of Americans edged up 0.3 percent. Meanwhile, industry output fell 1 percent during the month.

The 0.4 percent growth rate for the overall economy last quarter compared to a robust 6.9 percent annual rate in the final three months of 1978. In the first quarter of 1978, the economy's growth declined at an 0.1 percent pace. CAPTION: Picture, Treasury Secretary W. Michael Blumenthal, right, and Ford Motor Co. Chairman Henry Ford II talk in Detroit.AP