Colonic temperatures (TC), heart rates (HR), back skin and tail skin temperatures (TST) were measured in six warm acclimated (+24 °C) male albino rats running on a treadmill at three different work loads (HR ranging from 400 beats/min to 500 beats/min). Ambient temperatures (TA) ranged from about +8 °C to about +30 °C. TC increased immediately upon onset of work. Exercising in a cold environment ultimately made the rats hypothermic and in a warm environment hyperthermic. Within the limits set by the external thermal stress the rats controlled TC independently of the work intensity.High trunk skin temperatures were recorded in all experiments. Exercise in cold and cool environments produced tail skin vasoconstriction. In the 21 °C environment half of the rats produced tail skin vasodilation. In the 28 °C environment most experiments produced this effect. Cessation of work was accompanied by prompt vasoconstriction. The results indicated that exercise time before tail vasodilation was affected by exercise as well as by the tail skin temperature prior to vasodilation.