The purpose of the study was to assess the effect of time of day on the behavioral thermoregulatory patterns of nocturnal rodents, the Long-Evans (LE) rat, Fischer 344 (F344) rat, and the golden hamster. Individual animals were placed in a temperature gradient for 4 days while selected ambient temperature (ST(sub a)) and motor activity (MA) were monitored. Food was provided at the cold and warm ends of the gradient and water was provided ad lib. All animals eventually showed a 24-h rhythm of ST(sub a) and MA characterized by a preference for cooler T(sub a)s during the dark period which coincided with an increase in MA. Both rat strains had ST(sub a)s of about 28 C during the light period that decreased to 22-24 C during the dark period. The F344 rat developed a ST(sub a) rhythm by the second day in the gradient, whereas the LE strain required 4 days. The hamster exhibited relatively warm ST(sub a)s of 32-33 C during the light period that decreased to 26-28 C during the dark period. The nocturnal preference for cooler ST(sub a)s contradicts a current concept of an elevation in set point of the thermoregulatory system. However, the data also suggest that behavioral and autonomic thermoregulatory effectors may operate independently in the control of night time elevations in body temperature. (Copyright (c) 1993 Pergamon Press Ltd.)