Past studies have found that the laboratory rat placed in a temperature gradient prefers temperatures that are markedly below its lower critical ambient temperature (LCT), whereas other rodents (e.g., mouse, hamster, and guinea pig) generally select thermal environments associated with minimal metabolic expenditure. To further study the rat's thermoregulatory behavior, a temperature gradient was designed to monitor the selected ambient temperature (ST(sub a)) and motor activity (MA) of rats of the Long-Evans (LE), Fischer 344 (F344), and Sprague-Dawley (SD) strains over a 22 hr period. The new gradient differs from previous systems in two ways: (1) it is larger permitting more unrestricted movements and (2) it automatically monitors ST(sub a) over a relatively long, uninterrupted period of time. All three rat strains selected relatively cool ST(sub a)'s of 21 to 26C during the first 1 to 3 hr in the temperature gradient. This was followed by a gradual increase in the ST(sub a) which peaked at 4 (F344) to 6 hr (SD and LE) after being placed in the gradient.