In a series of 24 acute experiments on pentobarbital anesthetized dogs, thermal distributions generated by ultrasonic heating using a 1‐MHz PZT transducer were compared with intensity distributions mapped in a test tank. Relatively flat temperature distributions from 1 to 3 cm have been mapped in normal dog brain using “shaped” intensity distributions generated from ultrasonic emission patterns which are formed by the interaction between compressional, transverse, and flexural modes activated within the crystal. In contrast, these same intensity distributions generated marked variations in three malignant brain tumors presumably due to variations in tumor blood flow. The results of this study suggest that a practical clinical system for uniform heating of large tumor volumes with varying geometries is not an achievable goal. Our laboratory is developing a scanning ultrasonic rapid hyperthermia treatment system which will be able to sequentially heat small volumes of tumor tissue either to temperatures which will sterilize tumor or to a more conventional thermal dose. [Supported by NIH/NCI CA29731.]