1. The effects of protein-deficient and energy-deficient diets on the concentrations of skin glycosaminoglycans (GAGs) and collagen were studied in young rats. 2. In Expt 1, comparisons were made between animals fed on a control (C) diet (protein:energy (P:E) 0.210), a low-protein (LP) diet (P:E 0.032) and a very-low protein (VLP) diet (P:E 0.005) ad lib. Skin GAGs and collagen concentrations were determined at intervals as the animals became malnourished and when some of those fed on the VLP diet started showing signs of oedema. Concurrently, plasma albumin concentrations were determined to assess the extent of hypoalbuminaemia. With prolonged severe protein deficiency all GAGs species were reduced below normal levels. Collagen concentration decreased with prolonged protein deficiency. Protein-deficient rats developed hypoalbuminaemia. 3. In Expt 2, comparisons were made between animals fed on the C, LP and VLP diets ad lib. and others given the C diet in restricted amounts sufficient to maintain body-weights identical to those fed on LP or VLP diets ad lib. Energy-deficient rats, wasting at rates similar to protein-deficient ones, had higher skin GAGs concentrations than those in protein-deficient or control animals. With prolonged malnutrition, collagen concentrations were similar in protein- and energy-deficient animals and lower than those observed in control animals. Plasma albumin concentrations in energy-deficient animals were above starting values, except in severely energy-deficient animals at the end of the experiment. Protein-deficient animals developed hypoalbuminaemia. 4. In conclusion, the present study has shown that the main difference between prolonged severe protein deficiency and energy deficiency in the rat is that skin hyaluronic acid concentration is higher than normal in energy deficiency but below normal levels in prolonged severe protein deficiency. Initially skin collagen concentrations are higher than normal in both protein- and energy-deficient animals. However, with prolonged malnutrition the concentrations are reduced.