The body temperature of the domestic cat was determined by radiotelemetry from implants in the peritoneal cavity. Longitudinal data were collected and analyzed with a periodic regression technique. Thermograms and actograms were constructed in order to study the transients that follow a phase shift of the LD cycle and to discern the freerun in body temperature and activity in constant dark. The main results indicate that the domestic cat has a prominent circadian rhythm in body temperature. Prominent transients occur after a phase shift of the LD cycle, and the temperature rhythm freeruns in constant dark, maintaining a constant phase angle relationship with the freerunning rhythm in activity.