1. Colonies of rats have been maintained for twleve generations on diets adequate (dietary protein energy: total metabolizable energy (NDp: E) 0-1) or marginally deficient in protein (NDp: E 0-068). 2. In the malnourished colony, the proportion of "small-for-gestational-age" offspring was ten times as high as amongst the well-nourished colony, growth was slow, sexual maturation delayed, especially in the females, and, when adult, both sexes were significantly lighter and shorter than adults of the well-nourished colony. Organs, other than the eye, weighed less than those of well-nourished "age" controls, but when expressed relative to body-weight, the brain, pituitary, thyroid, adrenals, testes, thymus and eyes were larger, the pancreas unchanged and the kidneys smaller than those of the well-nourished "age" controls. The relative weight of the liver showed little change, being slightly increased in the males and, like the ovaries, slightly reduced in the females. On a body-weight basis, the brains were about 50 percent heavier than normal, but in absolute terms were 5-5-5 percent lighter than those of the well-nourished animals, the cerebellum (10-5 percent lighter in males and 12-9 percent lighter in females) being more severely affected than the cerebrum (4 percent lighter). 3. The young malnourished rats showed increased exploratory activity, transient head tremors and an increased sensitivity to noises, the latter being long-lasting if not permanent. When adult, they showed marked differences in behaviour and learning patterns and it was difficult to attract and hold their attention. In situations demanding a choice the animals were very excited, emitted loud squeals and tried to escape from what was clearly a stressful situation. However, a casual examination of the malnourished adults revealed a rather small, badly groomed, excitable rat without gross abnormalities. 4. The findings are discussed in relation to changes found in malnourished human communities.