WASHINGTON (AP) _ Americans' rate of borrowing rose in April, fueled by heavy credit card use.

Consumers' credit outstanding, excluding mortgage debt, advanced at a 3.4 percent annual rate in April to a seasonally adjusted $1.33 trillion, the Federal Reserve said today.

But the increase in the annualized rate of borrowing in April was the smallest so far this year and marked the third straight month that the rate of borrowing has slowed.

In March, American consumers borrowed at an annual rate of 4.6 percent, which was much higher than the Federal Reserve had estimated. In February, the annualized borrowing rate was 9.1 percent, and in January the annualized rate was 15.3 percent, according to the Fed's revised figures.

For April, revolving credit _ primarily credit cards _ showed the strongest increase: 6.8 percent at an annual rate. Auto loans increased by a 5.8 percent annual rate. And borrowing lumped into a catchall category that includes education loans, vacations, mobile homes and boats declined by 7 percent.

All told, consumers' borrowed $3.7 billion more in April than during the month before.

The tapering off of borrowing increases this year follows strong consumer borrowing in 1998.

And although the increases in the rate of borrowing are diminishing, consumers' confidence in the economy appears to be high.