The government's main gauge of future economic activity showed a healthy increase in February for the second month in a row, an indication that the economy may not be slowing down as much as thought.

The Commerce Department said yesterday its index of leading indicators rose 0.7 percent last month. The increase for January was revised slightly to 1.5 percent. The turn upward follows a period of fluctuation in the index, including four months in the last nine when the indicators actually fell.

Commerce also announced another piece of positive economic news: Sales of new single-family homes rose 6.2 percent last month after falling 0.5 percent in January. The rebound left sales at a seasonally adjusted annual rate of 638,000.

The increase in the leading-indicator index was due mainly to a strong rise in inflation-adjusted orders for plant and equipment, a sign that national output may go up a bit more during the first three months of the year than the 2.1 percent rate forecast by the government last week.

Commerce Secretary Malcolm Baldrige said the rise in orders was "a sign that capital spending will again be a driving force in the economic expansion." White House spokesman Larry Speakes said, "With the coming of spring, the indicators signal renewed optimism."

Baldrige noted, however, that the indicators would have to go up by 0.5 percent per month for the administration to feel comfortable with its 4 percent real growth rate projection for this year. The gains during the last six months have averaged 0.3 percent.

Economists also pointed out that the indicators have been sliding up and down in a very inconsistent fashion since they turned downward for the first time in 21 months last June.

"Where the monthly indicators are of use is where they show a trend. They've been up and down lately, which means there is no trend," said David W. Berson, senior financial economist with Wharton Econometrics.

GNP will probably grow by at least an annualized 3 percent during the first quarter, but growth is expected to remain slower than the 4.3 percent rate registered in the fourth quarter of 1984 or the 6.9 percent rate for the entire year, Berson said.

Jerry Jasinowski, chief economist for the National Association of Manufacturers, said the indicators' behavior implied that growth would speed up gradually during the year. Only the trade sector, which Jasinowski described as "hemorrhaging," would be a drag on the economy. Commerce announced Thursday that the merchandise trade deficit for last month was $11.4 billion, the largest since September.

The leading indicators, unlike most other economic statistics, don't measure one particular development. Instead, the index is a compilation of 12 indicators that, taken together, have a good track record in predicting economic performance.

In February, five of them went up, five went down and two were not available. The rising components were the plant and equipment orders -- which had declined in January but alone almost entirely offset the negative components in February -- stock prices, real money supply, net business formations and the pace of delivery of goods to companies.

The declining components were the length of the average workweek -- which analysts attributed to bad weather -- real business orders for consumer goods and materials, initial claims for unemployment insurance, changes in prices of sensitive materials and building permits. Inventory changes and changes in consumer and business borrowing were not available.