The ability of a modestly hypothermic victim to exercise, and indeed shiver, and thereby generate sufficient heat to rewarm has been questioned. One fit healthy subject was cooled in stirred water (7.7 degrees C) to a core temperature (Tc) of 32 degrees C. Tc by esophageal probe, cardiac rhythm, blood pressure (BP) and oxygen consumption (VO2) were monitored pre-, intra-, and postcooling (exercise). The subject rewarmed spontaneously as well as by exercising on a treadmill at speeds from 0.7 to 3.5 mph. Amount of Tc afterdrop (AD) was measured and rate of Tc increase (RI) and exercise and shivering contributions to heat production were calculated. The AD was 0.8 degrees C and the RI was 5.2 degrees C.h-1. VO2 values at the onset of hypothermic exercise indicated an approximate 4-fold increase in heat production from the normothermic resting value. A small portion of this heat production resulted from exercise while the majority was from shivering. Maximal heat production occurred at the lowest Tc (31.2 degrees C.). This represented an approximate 5-fold increase over the resting normothermic value. Shivering continued to provide the majority of the heat. As Tc increased, however, although heat production remained relatively constant, the contribution of exercise increased and that of shivering decreased. It was possible for this man to do a slight exercise at a Tc as low as 31.2 degrees C and the rewarming rate from shivering was substantial. Significant heat production is initially primarily by shivering thermogenesis, but soon, at a higher Tc, by exercise as well.