A large (150×150×40 cm) granite sample, sawn diagonally in half to simulate a fault, was deformed in a biaxial rock press at normal stress up to 6.41 MPa. Displacements and local shear stress were monitored along the fault (200×40 cm). Temperature transients as large as 10 m°C were recorded following stick slip events at distances of 0.2 to 1.0 cm from the fault and were related to stress, displacement, and total work. The temperature measurements were used to calculate the heat generated on the fault during slip. Frictional heating was found to be 94±2% of the total work expended in each event, implying a seismic efficiency of 4–8%. When water was injected onto the fault, both fractional stress drop and static frictional stress increased. The efficiency of frictional heating was not lowered by the presence of water. Heat generated during deformation of a 0.15-cm-thick layer of simulated gouge was also measured for sliding rates from 0.09 to 9.1 μm/s. In these gouge experiments, temperature rises were less than 0.1°C and were proportional to sliding rate.