American business built up new inventories at a somewhat slower rate in February, but stockpiling still continued at an uncomfortably fast pace, underscoring fears that the economy may be overheating.

Commerce Department figures showed that the book value of manufacturing and trade inventories rose $3.5 billion, or 0.9 percent, in February following a sharp 1.2 percent jump in January. The increase brought inventories to an end-of-month level of $387.5 billion.

The buildup was heavily concentrated in the manufacturing sector, where inventory levels leaped 1.3 percent in February-virtually the same as January's rise of 1.4 percent. Inexplicably, retail inventories declined 0.7 percent after climbing 0.9 percent in January.

With sales and manufacturing shipments up about 1 percent from January levels, the closely-watched inventory-to-sales ratio continued to show business stocks equal to 1.41 months' sales-unchanged from January. Any unusually sharp rise would have sent policymakers worrying.

William A. Cox, the Commerce Department's deputy chief economist, cautioned against reading too much into the figures. Cox pointed out that the book value of inventories has been bloated by inflation in recent months, and that actual volume of inventories did not rise that much.

Nevertheless, the overall figures appeared to be consistent with contentions of top administration economic advisers that the economy may be overheating, particularly in the industrial sector. Wholesale inventories soared a sharp 2 percent-double their increase in January.

The Carter administration is considering some tightening moves to help cool the economy including asking the Federal Reserve Board to raise interest rates or to impose credit controls. Insiders say Carter is expected to make a decision on a possible change within one or two weeks.