-Exogenous ¡3-endorphin or the opioid antagonist, naloxone, was added to the culture medium of control or LH-treated granulosa cells to elucidate the role of (3-endorphin in follicular steroidogenesis. ¡3-endorphin decreased basal progesterone (P 4 ) production by Gc isolated from small and medium follicles and had no effect on P 4 production by cells from large follicles. (3-endorphin decreased P 4 production by LH-treated Gc isolated from small, medium and large follicles. Naloxone increased basal P 4 production by cells from small and medium follicles and had no effect on P 4 production by Gc from large follicles. Neither agent had any effect on basal oestradiol (E 2 ) secretion by Gc from any of the follicles. However, E 2 secretion by LHtreated Gc from all sizes of follicles was inhibited by (3-endorphin, with the greatest effect on cells from small follicles. We conclude that opioid peptides act in the follicle in a paracrine manner to prevent excessive basal progesterone secretion by Gc and may also be involved in