This paper examines the role of hormones in the normal responses of muscle protein synthesis to nutrient intake and the use of hormones to improve the effects of nutritional therapies in patients with protein-wasting conditions. In growing rats, the increase in muscle protein synthesis after feeding seems to be mediated by the rise in plasma insulin and also by an enhanced sensitivity of the muscle to insulin brought about by the amino acid leucine. In adult rats, however, the responsiveness of muscle to both feeding and insulin is much reduced, suggesting that changes in protein degradation play an important role in the response to feeding. Similarly, in adult humans, muscle protein synthesis is not affected by insulin, but is stimulated by insulin-like growth factor (IGF)-I and growth hormone (GH). The effect of GH treatment has been studied in a number of different groups of patients suffering from protein wasting, and improvements in nitrogen balance and lean body mass have been reported. In a study of patients with acquired immunodeficiency syndrome (AIDS), however, GH treatment for 2 wk caused a fall in muscle protein synthesis in the patients with wasting, despite an increase in healthy controls, suggesting that the responsiveness of muscle to the hormone may be altered by the stage of the disease.