A comparison is made of the temperature- and water-relations of the day-active skink Mabuya quinquetaeniatus and nocturnal gecko Tarentola annularis. In darkness with regularly fluctuating temperatures M. Quinquetaeniatus is active during hot periods, T. annularis when the temperature falls. The lethal temperatures of both species for 24 hours exposure are around 40°C, but T. annularis can withstand a considerably higher degree of desiccation which may be correlated with its comparative lack of mobility. High sand temperatures are not avoided by either species until body temperatures rise. It is postulated that if heat-death results from physiological oxygen deficiency, as previously suggested, some other cause, such as protein coagulation, must operate simulataneously.