Results of physiological studies from some ten U.K. Human Adaptability projects are presented. U.K. investigators made major contributions in developing and adapting techniques for the assessment under field conditions of work capacity, heat tolerance and respiratory function. The various ethnic studies of work capacity revealed the special role of body size and muscularity, as well as training, in determining the observed inter- and intra-population variance. The results on samples from U.K., New Guinea, the Caribbean, Israel, West and East Africa and the Ethiopian highlands gave no indication that genetic differences were significant in determining population differences. Differences in heat tolerance reflect in general the intensity of heat exposure, especially when combined with hard physical work. Indigenous peoples in Africa and New Guinea show some modification in sweating responses which do not appear to be genetically determined but are in some way, as yet not clearly established, attributable to long continued residence in tropical climates. In renal function the desert Bedouin show no special adaptations. Successful adaptation for life in hot regions imposes changes in way of life as well as in dietary intake, particularly of salt and water, as shown by studies in the Sudan and Tanzania. Lung function of some seven ethnic groups were analysed in terms of lung volume bellows function, gas exchange and responses to exercise and carbon dioxide. The relative importance of genetic and non-genetic factors was examined.