WASHINGTON (AP) _ The nation's productivity, a key gauge of future prosperity, shot up at an annual rate of 4 percent in the first three months of this year.

The Labor Department said today that the gain in productivity, the amount of output per hour of work, in the first three months of this year followed a solid 4.6 percent increase in the fourth quarter of 1998.

The first-quarter increase was even better than the 3 percent advance many economists had been expecting and indicated a spurt of improved performance in productivity that was first noticed two years ago is continuing.