Porcine follicular fluid contains at least two factors which can modify porcine granulosa cell function in vitro. One factor, found primarily in fluid from 1--2 mm follicles inhibits LH stimulated cyclic AMP accumulation, basal and LH stimulated progesterone secretion, and morphological luteinization of granulosa cells from medium sized (3--5 mm) and large (6--12 mm) follicles. Following inhibition by the follicular fluid luteinization inhibitor, incubation with both OH and a phosphodiesterase inhibitor partially restores cyclic AMP accumulation and fully restores progesterone secretion. This suggests that the defect induced by the luteinization inhibitor is primarily an alteration of cyclic nucleotide metabolism. Another factor found primarily in fluid from large preovulatory follicles stimulates basal and gonadotrophin stimulated progesterone secretion by granulosa cells from small antral follicles. We hypothesize that it may be necessary for the fluid within an individual follicle to shift from being inhibitory of luteinization to being permissive or stimulatory for the granulosa cells to mature rather than become atretic.