Temperature regulation has been studied in 34 Yemenite Jews and 33 Kurdish Jews, including both men and women, using a standard thermoregulatory function test based on an air conditioned bed. The subjects were first tested at the Negev Institute for Arid Zone Research in the hot summer of 1968 and a proportion were then retested 6 months later in the cooler winter weather. The results reveal no differences between the Kurdish Jews and Yemenite Jews in thermoregulatory function. The women of both ethnic groups had much lower sweat rates, stored more heat before sweating was initiated and had higher skin temperatures at sweat onset than the men. These sex differences are discussed in relation to previous studies on the differences in temperature regulation between European men and women. Both males and females sweated less in the winter than in the summer. In the summer experiments, sweating capacity corresponded to the level found in partially heat acclimatized Europeans, whereas in the winter experiments the level for the Israeli Jews was significantly lower than that for unacclimatized European controls tested in Britain.