Paired groups of rats whose growth rates were restricted to similar degrees by feeding either protein- or energy-deficient diets had very different patterns of nitrogen metabolism. The protein-deficient animals showed evidence of dysadaptation in that liver protein was much more reduced than in the energy-deficient ones, whereas total muscle protein was higher. Plasma albumin concentrations decreased only in animals fed the low protein diet; with energy restriction a small but significant rise in albumin was seen. Although plasma amino acid concentration fell in both protein- and energy-restricted groups, a distorted pattern, with a markedly raised non-essential:essential amino acid ratio was found only in the protein-deficient rats. These differences in nitrogen metabolism could not be explained on the basis of variations in the plasma concentrations of insulin, corticosterone or growth hormone. The relationship of these changes to the development of kwashiorkor in man, and the use of plasma albumin and amino acid as indicators of inappropriate internal distribution of protein, resulting in dysadaptation, are discussed.