Follicle maturation and acquisition of developmental competence by the oocyte require an interplay between granulosa cells and the oocyte, involving Growth Differentiation Factor 9 (GDF9) and Bone Morphogenetic Protein 15 (BMP15) mediated modulation of follicular steroidogenesis. In the horse, dynamic intrafollicular concentrations of cortisol were associated with follicle growth and oocyte maturation, suggesting a functional relationship between cortisol and oocyte-derived factors. The aim of this study was to analyze effects of cortisol on steroidogenic activity, apoptosis and proliferative capacity of equine granulosa cells, as well as the relationship with the oocyte-derived factor GDF9. For this purpose, granulosa cells were collected three times in four mares each during the breeding season by transvaginal aspiration in the absence of a dominant follicle. Cells collected on the same day were pooled and then seeded on 12-well plates to grow to 70-80% confluence before being stimulated with 100 ng/ml IGF1 for 24h, in the presence or absence of cortisol in increasing concentrations: 50, 100 and 150 ng/ml. These concentrations were previously observed in antral follicles before deviation (50 ng/ml) and after deviation (120 ng/ml). In blocking experiments, cells were preincubated with 20 uM H89 (protein kinase A inhibitor). All experiments were repeated three times. GraphPad software was used for statistical analysis with one-way analysis of variance (ANOVA), followed by non-parametric multiple comparison post-hoc test.Expression of Star was similar in all groups (P>0.05), independent of cortisol concentration and was decreased (P<0.05) after preincubation with H89. Transcriptional levels of HSD3B2 were higher (P<0.001) after adding IGF1 and cortisol simultaneously, but not separately. The rise in HSD3B2 levels was independent of cortisol dose used (P>0.05). Blocking of Star with H89 increased GDF9 expression (P<0.01), both in the presence and absence of cortisol. Interestingly, addition of cortisol alone in all analyzed dosages raised BCL2 levels (P<0.05) and this effect was inhibited by H89 (P<0.05). However, there was no effect on CASP3 levels (P>0.05), regardless of treatment used. A higher proliferating capability of equine granulosa cells, as assessed by PCNA levels, was observed in response to IGF1 treatment (P<0.05), but only in the presence of 150 ng/ml cortisol. Despite apparently elevated, the expression of Ki67 marker in response to IGF1 in the presence of cortisol proved to be not significant (P>0.05). Taken together, our results demonstrate enhanced steroidogenesis as well as reduced apoptosis in equine granulosa cells in response to cortisol. Moreover, reduced steroidogenic activity (i.e. decreased expression of genes involved in progesterone synthesis) of granulosa cells increased GDF9 expression, which has previously been shown to be detrimental for the acquisition of developmental competence by the oocyte.