Mean skin temperatures, rectal temperatures and sweat rates were measured on four highly acclimatised subjects. They were exposed for 180 min to 16 different combinations of 4 metabolic rates and 4 air temperatures, ranging from cold (10°C) to hot (49°C). Mean sweat rates for the second hour were plotted against mean rectal temperaturesfor 4 different levels of skin temperature and viceversa. These graphs indicate, firstly, that sweat rate does not increase until rectal temperature rises above a threshold value of 36.5°C; thereafter the increase in sweat rate depends upon the level of mean skin temperature, being greater the higher the mean skin temperature is. Secondly, sweat rate does not increase markedly until mean skin temperature rises above 33°C but the increase in sweat rate above 33°C depends upon the level of rectal temperature, being greater the higher the rectal temperature is. The interrelated effects upon sweat rate of mean skin temperature and core temperature can be explained by means of a relatively simple physiological system, comprising temperature sensors in the hypothalamus and skin, and a heat loss and a heat conservation integrating centre. Mathematical equations have been developed to express the relationships between sweat rate, core, and mean skin temperature for cool conditions, but these relationships are so complex that it is preferable to use an analogue computer for these purposes.