Industrial output dropped by 0.4 percent last month, adding to evidence that the economy may have declined during the summer, the Federal Reserve Board reported yesterday.

High interest rates are mainly responsible for the economy's downturn, analysts say.

The drop in output in the nation's factories was only the second recorded this year, and followed an output rise of 0.3 percent in July. Most of the August decline was caused by a fall in auto production, the Federal Reserve reported. In addition there was a sharp cutback in appliance production in the month.

Yesterday's figures come after official reports this week showing weaker retail sales and the beginnings of a buildup in inventories of unsold goods. These reports together suggest that the economy has declined in the current quarter after a 2 1/2 percent drop in real output in the second quarter of the year.

However, experts generally have been surprised by the economy's strength this year. A long period of record-high interest rates has not yet toppled the United States into a serious recession, and many analysts believe that, although the economy may be sluggish, it will not fall sharply in coming months.

"The big question" about the fall in industrial production is "does this signal a big drop: is the recession really here?" Otto Eckstein, president of Data Resources Inc., a forecasting company, said yesterday. "The answer is 'still not yet, not yet'," he added. "But if interest rates stay high another few months, it's recession for sure."

Auto production, at an annual rate of 6.5 million units, was below July's rate by more than 10 percent, the Fed reporter. Production of small trucks was down even more sharply.

Consumer goods output fell by 1 percent during August under the impact of the decline in production of autos and home appliances. Output of all home goods was down by 1.8 percetn in the month.

Business equipmment production was up by one-half percent in August, continuing a run of slight increases. Reductions in the output of steel and parts for consumer goods contributed to a one-half percent drop in materials production.