Nutritional intake is an important regulator of plasma somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) concentrations in plasma. Concentrations in humans are reduced to the hypopituitary range by fasting for only a few days, and their normalization after fasting depends on the adequacy of energy and protein in the refeeding diet. The changes correlate with changes in nitrogen balance. In rats we have observed a close relationship between change in plasma Sm-C/IGF-I and hepatic GH binding with fasting and refeeding, suggesting that alterations in GH binding might be responsible partly for changes in Sm-C/IGF-I. When malnourished humans are given nutrient repletion, the increase in Sm-C/IGF-I is far more dramatic than changes in other nutrient-related serum proteins.