Comparison of thermal exchanges in men and women under neutral and hot conditions.

1. The thermoregulatory responses in unacclimatized men and women were analysed and compared by means of standard heat exposure tests which allowed evaporative losses, body temperatures, heat storage and the complete thermal balance to be continuously recorded in all subjects. 2. The most pronounced differences were observed in delay before onset of sweating. Sweating always occurred faster with lower body temperatures in men than in women. The period immediately following ovulation was characterized by an increase in onset delay and a decrease in the sensitivity in sweating response compared with the pre‐ovulation period. 3. The evaporative rate in the steady state did not change significantly in the post‐ovulation period and was found to be higher in men who consequently had lower mean skin temperatures. Skin conductances, different under thermo‐neutral conditions, were the same in men and women under hot conditions. 4. Women showed a more definite increase of body temperatures and heat storage than men due to delayed sweating and decreased sweating sensitivity. The body heat content in the steady state increased more markedly in women than in men. Furthermore, the highest heat storage values were found during the post‐ovulation period. A high degree of correlation was found between body heat content and absolute onset delay. 5. The sweating kinetics and the transient phase just before the steady state of the thermal balance appeared to be decisive factors in the differentiation of the thermoregulatory behaviour in women before and after ovulation. Heat storage achieved during the transient phase and not made up by adequate evaporation seemed to be the cause of the large increase of body temperatures and heat content shown by women in hot environments.