1. Preferred body temperature of five diurnal, Psammophis philipsii and three nocturnal, Lamprophis fuliginosus, snakes was measured in a thermal gradient chamber by indwelling colonic thermocouples, before and after injection of a variety of pyrogens. 2. The snakes achieved their preferred body temperature by moving up and down in the gradient chamber; it was about 33 degrees C for P. phillipsii and 25 degrees C for L. fuliginosus. 3. The snakes did not develop fever in response to any of the pyrogens, whether gram-negative or gram-positive in origin, either on the day of injection or on the subsequent day. 4. We believe that fever is rare amongst reptiles.