The study was conducted to correlate the preferred thermal environment of the unrestrained guinea pig with the activity of its thermoregulatory effectors when maintained under a wide range of ambient temperatures (Ta). Eight male guinea pigs were used in a series of experiments on behavioral and autonomic thermoregulatory function. In the behavioral experiment, individual guinea pigs were placed in a temperature gradient for 90 min while their position in the gradient was noted at 5 min intervals during the last 30 min of treatment. Their position in the gradient corresponded to a preferred Ta of 30.6 +/3.8 (S.D.) C. In the experiments to determine autonomic function, individual guinea pigs were placed in an environmental chamber thermostabilized to Ta's of 16 to 34 C. Metabolic rate (MR) and evaporative water loss (EWL) were continuously monitored for 90 min. After the guinea pigs were in the chamber for 90 min their colonic temperature was measured. MR was relatively stable between Ta's of 20 to 34 C.