To provide the basic information needed for studies of drug toxicity in rats suffering from protein deficiency, weanling male albino rats were fed for 28 days on either laboratory chow or a synthetic diet containing 27% or 8% of casein. The low-protein diet produced a kwashiorkoric cachexia characterized by stunting, alopecia, tail dermatitis, apathy, augmented calorie intake per kg body-weight and a 20% death rate. Autopsies at weekly intervals disclosed that the synthetic normal-protein diet produced a slight loss of total body-weight, due to loss of weight in the adrenal glands and gastrointestinal tissues, and a more-or-less generalized dehydration of body organs. The body-weight of the kwashiorkoric rats was half that of the control groups, due especially to loss of weight in caecum, kidneys, liver, muscle, skin, spleen, salivary glands and thymus gland; loss of weight was less marked in other organs, particularly the stomach, heart and testes and, especially, the brain. There was further dehydration of adrenals and brain but the liver and gastrointestinal tissues tended to be hydrated. The kwashiorkoric diet had not entirely suppressed growth-stimulated relative increase in weight of muscle, skin and testes. The histology of organs was essentially normal in the survivors. Absence of oedema in organs distinguished the kwashiorkoric rats from rats with total calorie depletion.