1. Thermophysiological responses of four men and four pre- and postovulatory women were compared in humid heat conditions. Responses of pre- and postovulatory women are similar except for body temperature levels, which were significantly higher after ovulation. 2. Pronounced sex-related differences were observed in sweating rate and in body temperature variations. For the same evaporation, the sweat rate in men was higher than in women; as a consequence of this, the dripping rate was larger in men and thus the sweat decline was more important. Body temperature increases were larger in men in function of time and therefore temperature regulation in women was considered to be more efficient.