High mortality rate in rats with large medial preoptic lesions discourage their use in studies of brain function. However, virtually all such animals (six out of seven) survived indefinitely if kept at an ambient temperature of 15 degrees C for 2 hours before and 10 to 12 hours after the lesions were made. Although these rats appeared otherwise healthy, they could not maintain normal both temperatures in short-term cold tests. In contrast, five of the nine rats kept at 25 degrees C died within 10 hours after the operation, and three more died within 5 days. Rats kept at 25 degrees C had a much higher incidence of cardiac arrhythmias than did rats kept at 15 degrees C, which may be responsible for their higher moratlity rates.