Growth hormone (GH) has previously been reported to influence the adipose conversion of 3T3-F442A murine fibroblasts, partly by causing these cells to exit the cell cycle and to become unresponsive to serum-stimulated mitogenesis. To better understand this process, quiescent fibroblasts were treated with fully stimulatory doses (50 nM) of epidermal growth factor (EGF) in the presence or absence of pituitary human GH (hGH) or the phorbol ester phorbol 12-myristate 13-acetate (PMA), which is known to down-regulate EGF receptor activity. EGF-induced DNA synthesis was attenuated by hGH in a dose-dependent manner with an ED 50 of approximately 0.1 nM and a maximally effective dose of 10-30 nM. This effect appeared to be the result of inhibition of DNA synthesis and exclusive of a time shift in the initiation of the S phase of the cell cycle. Additionally, insulin-like growth factor-1 (IGF-1), which can act as an important in vivo mediator of GH, failed to mimic the antimitogenic effects of GH. The ability of hGH to antagonize EGF-stimulated mitogenesis did not appear to be due to the down-regulation of EGF receptor mass or to pronounced changes in EGF-induced tyrosine kinase activity. Furthermore, when GH was administered at various times after EGF addition, GH continued to be effective at inhibiting EGF-induced DNA synthesis for up to 9 hr after EGF treatment. Modulation of EGF-induced cell cycle progression was further evidenced by the ability of GH to promote a marked decrease in the EGF-induced expression of D cyclins. In comparison, PMA inhibited EGF-induced DNA synthesis for up to 18 hr after EGF addition and also down-regulated EGF receptor mass and activity; these observations suggest that the mechanism of GH action is largely distinct from that of PMA. We conclude that GH can selectively and dose-dependently modulate EGF receptor-mediated DNA synthesis exclusive of any rapid or extensive effects on EGF receptor mass or tyrosine kinase activity. Furthermore, the capacity of GH to attenuate EGF-induced mitogenesis, even when administered 9 hr after EGF addition, and the GH modulation of EGF-induced expression of D cyclins, suggest that there are GHinduced effects on systems involved in the transition of these fibroblasts through the G 1 phase of the cell cycle. In sum, these data support a specific interaction of this somatotropic hormone/cytokine with EGF in the control of cell cycle progression in 3T3-F442A fibroblasts.