Levels of mRNA for the insulin-like growth factor-I (IGF-I) in rat heart and skeletal muscle and its dependence on GH were investigated using a solution hybridization assay. Levels of IGF-I mRNA decreased following hypophysectomy, and replacement therapy with human GH (hGH) normalized heart and skeletal muscle levels. The stimulatory effect of hGH was dose-dependent, the lowest effective dose being 100 micrograms. A significant increase of IGF-I mRNA was observed 60 min after s.c. administration of 100 micrograms hGH and the maximum increase was apparent 6-12 h after hGH injection. Administration of 200 micrograms IGF-I or 11 micrograms insulin did not significantly change levels of IGF-I mRNA. The results show that GH regulates the level of IGF-I mRNA in rat heart and skeletal muscle and give further support to the hypothesis that locally produced IGF-I might be a local mediator for the direct stimulatory effect of GH on the growth and development of heart and skeletal muscle.