A sample assembly has been designed for ultrasonic velocity measurements in rocks and crystals at elevated temperature and pressure. The new assembly can be used without coupling bonds for determining Vp and Vs. The assembly permits control of pore pressure with varying confining pressure and temperature for pores connected to the surface of the rock. Either pulse transmission time or pulse interferometric measurements can be made. Transducer delays are determined as a function of pressure and temperature by using the Z direction of single-crystal quartz as an acoustic standard. We have measured the pressure and temperature derivatives of a compressional wave interference frequency in the Z direction of quartz to 250¿C and 4 kbar using the phase comparison technique. At 25¿C and P=1 bar we find that F0=20.664¿0.0044 MHz, (∂F/∂T)P=-1.579¿0.0214 kHz/¿C, and (∂F/∂P)&tgr;=0.09729¿0.00125 MHz/kbar. The P and S velocity data as a function of pressure and temperature to 7 kbar and 300¿C are plotted for a Franciscan lawsonite metagraywacke core, illustrating the accuracy of pressure-temperature-velocity measurements in rocks.