Thirteen obese and 13 lean women were each exposed to a hot-humid condition (Tdb = 47 degrees C, Twb = 39 degrees C) for 1 hr. Measurements included rates of heart beat, respiration, sweat evaporation, and resting metabolism, and the rectal and mean skin temperatures. Resting metabolic rate was calculated from oxygen consumption and carbon dioxide production. The obese subjects were more tolerant of heat than were lean subjects, as indicated by the smaller thermal and metabolic responses of the obese.