The behavior of a quartz monzonite rock mass subjected to a thermal load from emplaced canisters with electric heaters simulating high‐level nuclear waste was monitored by a cross‐hole seismic technique in a drift 340 m below the surface in the Stripa mine facility in Sweden. Traveltimes and amplitudes of 20 to 60 kHz ultrasonic compressional (P) and shear (S) waves were measured over the experiment duration of 750 days, on 2 to 4 m paths between four diamond‐drilled boreholes around a heater. The signals were transmitted between the boreholes in six different directions and at different depths. Path‐averaged P- and S-wave velocities were obtained from the times of flight of pulses of acoustic waves between separate P- and S-wave piezoelectric transmitter and receiver crystals. The attenuation [Formula: see text] was obtained by a spectral ratio technique. When the heater was turned on, the P- and S-wave velocities increased to 4 and 10 percent, respectively, and stabilized at the elevated values. The P-wave velocities along a particular profile were found to increase linearly with the mean temperatures in the profiles sampled. These mean temperatures increased 25° to 55 °C during the course of the experiment. When the heater was turned off after 398 days of heating, the velocities decreased with temperature and finally reached levels in most cases below those observed prior to heating the rock. The highest thermal stress close to the heater was estimated at 55 MPa. [Formula: see text] values increased throughout the heating interval, reaching changes of up to 60 percent shortly after turning off the heater. The Q values reveal no direct correlation with temperature or the closely associated thermal stress, although these phenomena clearly are driving the variation in Q. There is strong evidence relating attenuation properties to fracture closure and pore pressure changes associated with draining of the rock mass.