A study has been made of three neighbouring populations living at 1500, 3000 and 3700 m in the northern Simien of Ethiopia. The environments of these populations not only differ in many climatic elements, but also probably in nutritional factors and exposure to infections. The growth and physique of the people vary with altitude and the lowlanders (at 1500 m) tend to have a more linear body build. Differences in chest dimensions can be related to functional differences in respiratory physiology, since the highland groups, both male and female, have larger forced expiratory volumes and forced vital capacities as compared with the lowlanders. The relationships between these measures of respiratory function and age, stature and weight also tend to be dependent on altitude, but in all the Ethiopian groups there is a closer relationship between body weight and respiratory capacity than in other populations. This distinctiveness is probably due to the characteristics of Ethiopian physique. A slight polycythaemia and elevated packed cell volume are evident in the highland groups but, unexpectedly, there is some evidence that at least at the time of the expedition the haemoglobin concentrations were lower. The highlanders also show a raised systolic blood pressure. Blood-group and demographic data suggest that the various populations are probably genetically very similar, and the findings are discussed in terms of physiological and developmental adaptability.