Concurrent with follicular development, the production of gonadal steroids takes place in a three-cell model in which progesterone is mainly secreted by granulosa, oestradiol by the external theca and testosterone by the inner theca cells (Porter et al., 1989). Progesterone (P4) (Wilson and Sharp, 1976a) and estradiol (E2) (Wilson and Sharp, 1976b and Kawashima et al., 1979) have known actions in the induction of the preovulatory LH surge.