Progesterone (P4) inhibits granulosa cell and spontaneously immortalized granulosa cell (SIGC) apoptosis by regulating membrane-initiated events. However, the nature of the signal transduction pathway that is induced by these membrane-initiated events has not been defined. To gain insights into the P4-regulated signal transduction pathway, mouse granulosa cells and SIGCs were cultured with 8-br-cGMP and P4. In culture, 8-br-cGMP mimicked P4's antiapoptotic actions. Because cGMP activates protein kinase G (PKG), the effect of PKG antagonists on P4-regulated SIGC viability was assessed. P4's antiapoptotic action was attenuated by the PKG inhibitors, Rp-8-pCPT-cGMP, KT5823, the PKG-1α-specific inhibitor, DT-3, and a dominant negative PKG-1α. Further, the type I isoform of PKG was shown to be expressed by SIGCs and activated by P4. P4's antiapoptotic action was not affected by the PKA inhibitor, KT5720. Collectively, these findings indicate that P4 maintains SIGC viability by activating PKG-1α. PKG-1α-GFP was shown to localize predominantly to the cytoplasm of SIGCs. To identify potential cytoplasmic targets of PKG-1α, SIGCs were cultured for 5 h with P4 in the presence or absence of DT-3. Cell lysates were prepared and subjected to two-dimensional electrophoresis. The resulting gels were sequentially stained with ProQ-Diamond Gel Stain and Coomassie Blue to reveal phosphorylated proteins. The two-dimensional gels revealed one major protein, the phosphorylation status of which was abrogated by DT-3. Mass spectrometric analysis identified this protein as 14-3-3σ, with 14-3-3σ being phosphorylated on tyrosine 19, serine 28, serine 69, serine 74, threonine 90, threonine 98, and serine 116. Finally, difopein, a specific 14-3-3 inhibitor, was shown to induce apoptosis even in the presence of serum. These data suggest that 1) P4 regulates the phosphorylation status of 14-3-3σ through a PKG-dependent pathway and 2) 14-3-3σ plays a central and essential role in maintaining the viability of SIGCs.