In order to study the prevalence of hypertension and some of the factors relevant to its natural history, cross-sectional surveys were performed during the period 1967 to 1973 in five small Peruvian communities, two located at sea level and three above 13,000 feet of altitude. In total, 4,359 persons were studied at sea level (1,970 males and 2,389 females) and 3,055 at high altitude 2,189 males and 866 females). At high altitude, the age-adjusted prevalence of hypertension (particularly systolic) was definitely low; diastolic hypertension was more frequent in men than in women, and it was commoner than systolic hypertension. The reverse was observed in communities at sea level. Long-term blood pressure changes observed in natives accustomed to high altitudes but living at sea level, as well as in white persons usually living at sea level but residing at high altitude, appear to indicate that environmental forces are more important than genetic predispositions in determing the rarity of hypertension in the highlands. Among the environmental forces, chronic hypoxia seems to play an important causal role.