If trained to walk bipedally at a juvenile age and over periods of some months or years, Japanese monkeys gradually acquire a pronounced lordosis of the lumbar spine. This lordosis persists even in the ‘normal’, pronograde posture of these animals. It is due to a relative increase of the ventral lengths of the vertebral bodies. This morphological change is clearly an adaptation to the mechanical necessities of the upright body posture. Our result is in complete accordance with the development of a lordosis in human children between 1 and 5 years, as described recently by oth