Somatomedin preparations have previously been shown to suppress GH release. The effects of a synthetic recombinant human insulin-like growth factor analog (IGF-I; Thr 59) were, therefore, tested on long term GH secretion by male rat pituitary monolayer cell cultures grown in serum-free defined medium. IGF-I (3.25 nM) maximally suppressed basal GH secretion for up to 72 h by 66%, with an ED50 of 0.1 nM. Human pancreatic GRF-(1-44) (GHRH; 1 nM) stimulated GH secretion by 230% during 72 h. IGF-I (0.13 nM) suppressed GHRH-stimulated GH secretion by 30% (P less than 0.005). IGF-I (0.625 nM) completely abolished stimulation of GH by GHRH (1 nM), while higher doses of IGF-I (up to 6.5 nM) actually suppressed GH secretion even in the presence of GHRH (up to 1 nM). The depletion of intracellular GH levels in GHRH-treated cells was reversed by IGF-I (3.25 nM). PRL secretion was not altered in the same cells by IGF-I. Equivalent doses of epidermal growth factor and fibroblast growth factor did not alter basal or GHRH-stimulated GH secretion. Nitrocellulose dot hybridization of immobilized pituitary cell RNA extracts with rat GH [32P]cDNA showed that cellular GH mRNA levels were lowered in IGF-I-treated cells in a dose-dependent manner. Maximal suppression of GH mRNA was achieved with 0.65 nM IGF-I. IGF-I also inhibited the 3-fold stimulation of GH mRNA induced by 1 nM GHRH. The data show that IGF-I directly modulates GH gene expression at the level of the somatotroph by inhibiting basal and GHRH-stimulated GH secretion and GH mRNA levels during 72 h. These effects may occur at different postranscriptional sites. Alternatively, they may result from a direct inhibition of IGF-I on GH gene transcription.