Subjecting adult male rats to pinealectomy causes testicular hypertrophy, enhances the weight of the ventral prostate and of the seminal vesicles and increases pituitary LH [luteinizing hormone] stores. These data show that pinealectomy stimulates synthesis and release of pituitary LH and suggest that the pineal gland usually exerts an inhibitory influence on LH secretion. In order to determine whether such an effect of the pineal gland was taking place through a direct action on the pituitary gland or via the mediation of nervous structures, fragments of pineal tissue or crystals of melatonin (a neurohumor specifically synthesized in the gland) were implanted stereotaxically in the median eminence (ME), in the midbrain reticular substance (RS) and in the pituitary gland of castrated male rats. Placement of pineal fragments or of melatonin in the ME and in the RS is followed by a significant reduction of pituitary LH stores. ME and RS implants of melatonin also cause a reduction of plasma LH levels. Melatonin is unable to reduce pituitary LH when directly implanted in the gland. Pineal indole compounds may modify pituitary function by acting on receptors located in the ME and in the RS.