Laboratory experiments were performed where heat generation was measured during both stable sliding and stick-slip friction. For a given average slip velocity, stick-slip sliding produces heat at a considerably lower rate than stable sliding without appealing to high pore pressure or lubrication. These experiments show that faults can be apparently both strong when slip initiates and weak during rapid sliding, suggesting that different mechanisms operate under the two sliding conditions. If these results apply to the Earth, then all seismogenic faults should produce less heat than creeping fault zones. Possible explanations are that during rapid sliding, in the slip portion of a stick-slip cycle, either the friction coefficient or the normal stress is reduced by a factor up to about 7. Comparison with other experimental results suggests the reduction of normal stress by interface separation waves is the most likely explanation. ¿ 1998 American Geophysical Union