The present study sought to characterize the concerted action of FSH and insulin-like growth factor-1 (IGF-1) on functional differentiation of prepubertal rat ovarian granulosa cells in culture. To this end, we examined the regulation of three key genes encoding pivotal proteins required for progesterone biosynthesis, namely, side-chain cleavage cytochrome P450 (P450_scc), steroidogenic acute regulatory (StAR) protein, and 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD). Time-dependent expression profiles showed that P450_scc, StAR, and 3β-HSD gene products accumulate in chronic, acute, and constitutive patterns, respectively. Each of these genes responded to FSH and/or IGF-1 in a characteristic manner: A synergistic action of IGF-1 was indispensable for FSH induction of P450_scc mRNA and protein; IGF-1 did not affect FSH-mediated upregulation of StAR products; and IGF-1 alone was enough to promote expression of 3β-HSD. The responsiveness of the genes to IGF-1 correlated well with their apparent susceptibility to the inhibitory impact of tyrphostin AG18, a potent inhibitor of protein tyrosine kinase receptors. Thus, IGF-1-dependent P450_scc and 3β-HSD expression was completely arrested in the presence of AG18, whereas StAR expression was unaffected in the presence of tyrphostin. These findings suggest that FSH/cAMP signaling and IGF-1/tyrosine phosphorylation events are interwoven in rat ovarian cells undergoing functional differentiation. We also sought the mechanism of IGF-1 synergy with FSH. In this regard, our studies were unable to demonstrate a stabilizing effect of IGF-1 on P450_scc mRNA, nor could IGF-1 augment FSH-induced transcription examined using a proximal region of the P450_scc promoter (−379/ 6). Thus, the mechanism of IGF-1 and FSH synergy remains enigmatic and provides a major challenge for future studies.