While the earlier studies in the rabbit indicated that the posterior basal tuberal medial region of the hypothalamus exerts an inhibitory influence on the release of prolactin, we have now shown that hypothalamic implantation of perphenazine crystals can achieve prolactin release. Adult female rabbits which had been ovariectomized and primed with 0.1 mg/day estradiol for 10 days had their left abdominal mammary gland removed and a stainless steel tube carrying a perphenazine crystal was implanted stereotaxically in the median eminence of the hypothalamus. Examination of the mammary gland after autopsy on the 15th day showed marked mammary development with copious milk production in all experiments in which perphenazine was inserted in the median eminence. No mammary development was observed in the controls, either with perphenazine inserted in other sites of the brain, or with a phenothiazine derivative (aminopromazine), which does not initiate lactation, inserted in the median eminence, or with sham implantations of empty tubes. These experiments have shown that perphenazine initiates release of prolactin and lactation. It is assumed that this is due to a decrease in the hypo-thalamic prolactin-inhibiting activity of the median eminence.