1. Six healthy humans were immersed sequentially in baths maintained at a steady temperature of either 28 +/- 1 or 38-8 +/- 1 degree C. 2. Metabolic heat production was calculated by respiratory gas analysis. A ventilated capsule was placed on the forehead and sweat secretion was calculated from psychrometric recordings. Convective heat loss from one hand to water-perfused glove provided a continuous measurement of vasomotor response. 3. Heat production, sweating, and vasomotor heat loss were proportional to core temperature. 4. Sweating and vasomotor response were parallel. Vasoconstriction was complete, before the onset of shivering. 5. The thresholds for heat loss and heat production were superimposed, without a 'dead band' core temperature.