Serum levels of adrenal androgens were measured in children of both sexes living at sea level and at high altitude using a cross-sectional design. The levels were compared in relation to age at each altitude (150 m and 34300 m above sea level), and the first significant increase in mean levels compared with values at preceding ages was assessed and defined as adrenarche. A total of 118 boys and 95 girls aged 6-12 years living at low altitude and 95 boys and 104 girls aged 7-15 years living at high altitude, all of them attending public schools, were studied. Serum dehydroepiandrosterone, dehydroepiandrosterone sulphate and androstenedione were measured by radioimmunoassay and height and weight by standard anthropometric techniques. There were two significant increases in serum levels of adrenal androgens, the first observed in children between 6 and 8 years at sea level, and between 7 and 9 years at high altitude, and the second in children between 10 and 12 years at sea level and between 12 and 15 years at high altitude. Serum adrenal androgen levels increased earlier in children at sea level than at high altitude. It is concluded that adrenarche occurs later at high altitude than at sea level.