The index of leading indicators rose 0.4 percent in May and a previously reported 3.3 percent drop in April was revised to only a 2 percent decline, the Commerce Department reported yesterday.

An increase in the length of the average workweek in manufacturing was responsible for much of the rise, however, and that change largely reflected the end of the nationwide trucking strike in April, analysts said.

The index, on a quarterly basis, fell 0.3 percent from the fourth quarter to 1978 to the first quarter of this year. The average for April and May, at this point, is 1.5 percent below the first quarter average.

Many private economic forecasters believe the nation is either already in a recession or will be sometime soon, which would be consistent with the decline in the index that is seen when the erratic month-to-month changes are averaged over a quarter.

Three of the 10 indicators available for May increased: the workweek, new orders for consumer goods and materials received by manufacturers adjusted for inflation, and building permits.

Five of the 10 declined and two were unchanged. Dropping were contracts and orders for business plant and equipment adjusted for inflation, stock prices, the money supply adjusted for inflation, the change in total liquid assets, and the change in sensitive prices.

The layoff rate in manufacturing and the number of companies reporting slower deliveries from suppliers did not change.

The index of coincident indicators, which tends to move up or down simultaneously with economic activity, rose 0.3 percent in May.