*National Institute for Biological Standards and Control, Holly Hill, London, NW3 6RB and ‡Himalayan Rescue Association, P.O. Box 283, Kathmandu, Nepal (Received 23 May 1978) There is demographic evidence from studies in the Andes (James, 1966; Heer, 1967; Abelson, 1976) for lower fertility in high-altitude dwellers than in moderate- or low-altitude dwellers. In the Sherpas of Nepal, the completed fertility rate at high altitude (about 6·0) is appreciably less than that at lower altitudes (8·5; C. R. M. Bangham & J. M. Sacherer, unpublished observations). However, the physiological reasons (if any) for such a difference are obscure. The present study was carried out to test the hypothesis that such a fertility difference (between low- and high-altitude dwellers) is reflected in different serum concentrations of reproductive hormones. Sampling was carried out at altitudes of 4240 m in Khumbu, 2670 m in Pharak (south of Khumbu) and 1460 m in Kathmandu; the