Four groups of 10 young adult male rats of the Wistar strain were fed on a protein-free diet ad libitium for periods of 7, 28, 56, and 84 days. Control groups were fed a purified 20% casein diet. Food intake and body weights of rats were measured. Hemoglobin and plasma protein levels, weight, total nitrogen, and collagen nitrogen of skin, carcass, muscle, and liver were determined. Protein-deficient rats lost body weight and had low plasma protein concentrations, but hemoglobin levels remained normal until day 56 of deficiency. The liver lost weight and nitrogen more rapidly than the other organs; the severity of nitrogen depletion in the organs increased with time fed the protein-deficient diet. When protein deficiency was severe, collagen nitrogen concentration increased in organs and carcass. This was not due to an actual increase of collagen nitrogen content; comparisons among malnourished groups showed that the total amount of collagen nitrogen in carcass, liver, and muscle was maintained and that the amount in skin diminished as periods of protein deprivation increased. In control rats, results indicated that the amount of collagen nitrogen in skin, muscle, and carcass increased during growth. These results indicate that protein restriction in adult rats affects collagen metabolism, with skin collagen being more markedly affected than that of other tissues.