Data from the literature on hypothalamo-pituitary function in relation with sexual maturation have emphasized the role of several factors possibly involved in the modulation of LHRH and gonadotropin release. Here, the periodicity of LHRH signals seems to be very important. The mechanisms of hypothalamo-pituitary maturation at puberty seem to include the disappearance of an inhibitory control exerted by gonads on one hand and supra hypothalamic nervous system on the other hand. In relation with sexual maturation of male rats, we have studied the effects of estradiol upon the response of pituitary cells to LHRH in vitro, the effects of castration on hypothalamic LHRH and finally LHRH release from hypothalamus in vitro. Our observations suggest that the inhibitory controls are mainly effective through an hypothalamic action.