Several studies have suggested that the transcription factor GATA4 plays an important role in ovarian function. This study evaluated the effects of GATA4 on the regulation of the Cyp19 gene in primary rat granulosa cells under basal conditions and in response to stimulation by FSH. A significant increase in GATA4 mRNA, protein, and DNA binding activity was observed in rats treated with pregnant mare serum gonadotropin, a hormone that binds to the FSH receptors, and in granulosa cells incubated with FSH. Enrichment of the Cyp19 promoter was observed in granulosa cells treated with FSH after chromatin precipitation with an anti-GATA4 antibody. Mutation of the GATA binding site on the Cyp19 promoter and inhibition of GATA4 expression with specific small interfering RNA significantly reduced FSH-enhanced Cyp19 expression, whereas overexpression of GATA4 increased Cyp19 promoter activity. A synergistic effect observed between GATA4 overexpression and FSH treatment in Cyp19 expression was abolished by mutating Ser105 in the GATA4 protein or by pretreating granulosa cells with a protein kinase A inhibitor. Inhibition of phosphatidylinositol-dependent kinase (PI3-K)/casein kinase 2 or ERK1/2 attenuated GATA4/FSH synergism, whereas the simultaneous blockade of PI3-K/casein kinase 2 and ERK1/2 activity eliminated Cyp19 stimulation. Finally, we demonstrated that FSH increases GATA4 phosphorylation and that GATA4 activation requires the activation of multiple kinases, including ERK1/2, PI3-K, and protein kinase A. These findings demonstrate that GATA4 contributes in the regulation of Cyp19 expression in the rat ovary and provide the first evidence that FSH regulates GATA4 activity.