The possibility that the pineal exerts a regulatory influence on the gonadotrophic function of the adenohypophysis was investigated using in vitro techniques. Additionally, since a direct negative feedback of pituitary LH onto the hypothalamus occurred when adenohypophysial and hypothalamic tissues were incubated together, a continuous-flow system was devised to study the mechanisms of action of the pineal anti-gonadotrophic principles. Results demonstrated that the bovine pineal has marked anti-gonadotrophic properties in vitro. Data showed that the pineal hormones inhibited the release of the relevant hypothalamic release factor, thereby reducing secretion of LH from the adenohypophysis. Finally, results indicated that the pineal also stimulated the production by the hypothalamus of an LH-inhibitory factor.