Because of the potential for certain modalities of clinical ultrasound to cause tissue heating, it was deemed important to survey the literature for reports on temperatures and hyperthermia exposure durations at which biological effects occur in a wide spectrum of organisms. In general the higher the temperature or the longer the hyperthermia exposure, the greater the chance for observing a perturbation to the biological system. Special attention was given to ascertaining temperature exposure conditions below which no biological effects have been reported. The physical mechanism(s) by which heat produces biological effects is(are) not known but substantial quantities of empirically derived data have allowed for the calculation of exposure temperature-duration equivalences. It appears reasonably well established that short exposures to sharply-elevated temperatures result in a protective effect against further thermal insult; the generation of heat shock proteins by cells coincides with the onset of such "thermal protection." The information contained in this review indicates an absence of reported effects in animals below 39 degrees C.