Insulin-like growth factor I (IGF-I) suppresses GH gene transcription and GH secretion, while somatostatin (SRIF) only suppresses GH secretion. The interaction of these inhibitors of GH was, therefore, tested in primary rat pituitary cells grown in serum-free medium. Maximal inhibition of GH secretion (to 30% of the control value) was achieved by 13 nM IGF-I, while 5 nM SRIF suppressed GH to 36% of control secretion. The respective ED50 values for IGF-I and SRIF inhibition of GH secretion were similar (approximately 2.5 nM). Treatment of cells with the two agents together resulted in a further inhibition of basal GH secretion to 18% of control untreated cells (P less than 0.005). Increasing doses of SRIF (2.5-10 nM) in the presence of IGF-I caused a dose-dependent suppression of GH secretion. PRL levels were not altered by these treatments, indicating a selective effect on GH. GRH-induced GH secretion was further attenuated by combined IGF-I and SRIF treatment compared to the effect of either of these two agents alone. Northern analysis showed that IGF-I suppressed GH mRNA transcripts, while SRIF did not alter GH mRNA levels. The results indicate that physiological concentrations of both IGF-I and SRIF suppress long term basal GH secretion. Only IGF-I alters GH mRNA levels. These two peptides, therefore, appear to attenuate in vitro GH secretion by different mechanisms.