Two communities of Jews living in Israel were selected for study. They consisted of Jews from Kurdistan and from the Yemen. They lived in five villages in the Negev area of Israel where the climate is hot in the summer months and cool to warm in the winter months. Studies were carried out in the summer (June, July) and in the winter (January, February). The studies were confined to the age group 20 to 30, and 75% of the population in this age group were examined. The investigations comprised: (a) clinical examination, (b) anthropometric measurements, (c) genetic study of blood markers, (d) habitual activity, (e) energy expenditure, (f) food intake, (g) climatic exposure, (h) measurement of physical work capacity, and (i) reaction to a standardized heat test. The environmental conditions of the subjects were similar for the two groups and the majority were engaged in farming.