Sympathetic nerves innervate smooth muscle cells in the theca externa of Graafian follicles and induce contraction of the follicle wall. The interaction of prostaglandins (PGs) with the function of this neuromuscular complex has been elucidated by the use of isolated strips from the protruding part of the wall of bovine ovarian follicles. PGF2α contracts the strips, and it also potentiates the contractile response to electrical field stimulation of the nerves, but it has no effect on noradrenaline-induced contractions. PGE1 and PGE2 have direct relaxatory actions on the strip preparation (previously given an active contraction by carbamylcholine) and in high doses they, therefore, reduce the contractile response induced by noradrenaline. In low doses not affecting the noradrenaline response, PGE1 and PGE2 both decrease the amount of contraction induced by electric nerve stimulation, indicating an inhibitory action of the PGEs on the transmitter release. It is suggested that the role of PGs in the process of ovulation might, at least partly, be mediated by their interactions with smooth muscle cells and their sympathetic innervation in the wall of the Graafian follicle