Animal studies have shown that LH causes an increased synthesis of PGF by ovarian follicles both in vivo and in vitro. In lower animals, there is strong evidence that PGF acts as a mediator of certain LH effects, including maturation and extrusion of the ovum. Ovarian follicular tissue of 12 patients was halved or quartered and maintained in organ culture. Part of each follicle was exposed to gonadotropins throughout the culture period, while controls received no treatment. All tissues continued to synthesize PGF but in 11 cases the amount was greater in the gonadotropin-treated. It is concluded that PGF probably plays a role in human follicular function, including ovulation, and may act as a mediator of certain LH functions.