Experiments were conducted to test whether oestradiol (E2) protects granulosa cells from Fas ligand (FasL)-induced apoptosis and whether protection involves modulation of the cell cycle of proliferation. Treatment of cultured bovine granulosa cells with E2 decreased susceptibility to FasL-induced apoptosis. The effects of E2 were mediated through oestrogen receptor and were not mediated by stimulation of IGF production. E2 also increased the percentage of cells progressing from G1 to S phase of the cell cycle, and increased expression of cyclin D2 protein and the cell proliferation marker Ki67. Progression from G1 to S phase of the cell cycle was necessary for the protective effect of E2; blocking progression from G1 to S phase with the cdk2 inhibitor roscovitine, or blocking cells in S phase with hydroxyurea, prevented protection by E2. The stages of the cell cycle during which granulosa cells are susceptible to apoptosis were assessed. First, treatment with the G1 phase blocker, mimosine, protected cells from FasL-induced apoptosis, indicating that cells in G0 or early- to mid-G1 phase are relatively resistant to apoptosis. Secondly, examination of recent DNA synthesis by cells that became apoptotic indicated that apoptosis did not occur in S, G2 or M phases. Taken together, the experiments indicate that cells may be most susceptible to apoptosis at the transition from G1 to S phase. E2 stimulates transition from G1 to S phase and protects against apoptosis only when cell cycle progression is unperturbed.