To determine the mechanisms involved in the nutritional regulation of insulin-like growth factor I (IGF-I) production and action, we studied the regulation of IGF-I and IGF-Binding Protein 1 (IGFBP-1) gene expression by GH and amino acid availability in rat hepatocyte primary culture. Hepatocytes were isolated by in situ collagenase perfusion and cultured on Matrigel in serum-free medium containing insulin and hydrocortisone. Rat GH (500 ng/ml) increased IGF-I messenger RNA (mRNA) abundance 6.9-fold at 24 h, as measured by Northern Blot using an IGF-I-specific riboprobe. In contrast, IGFBP-1 mRNA levels were decreased by 41% after 24 h of rat GH treatment. Hepatocytes were incubated for 24 h in three media differing in their amino acid concentrations: 0.2X, 1X, and 5X the normal rat plasma concentration. Amino acid deprivation (0.2X) decreased the abundance of IGF-I mRNAs (-56% after 24 h), whereas amino acid excess (5X) increased it (+70%) in comparison to the 1X medium. In contrast, amino acid deprivation increased IGFBP-1 mRNA abundance (+69%), whereas excess decreased it (-75%). Studies of the interaction between GH and amino acids, accomplished by the simultaneous manipulation of the two, suggest that each factor modulates the IGF-I mRNA and the IGFBP-1 mRNA and protein response to the other. We conclude that the IGF-I and IGFBP-1 genes are regulated in opposite ways by GH and amino acid availability. Our observations suggest that amino acids and GH regulate the production of IGF-I directly and exert indirect effects on IGF-I action by regulating the production of IGFBP-1.