Destruction of the medial basal tuberal region of the hypothalamus of ovariectomized female New Zealand White rabbits whose mammary glands had been suitably developed led to initiation of milk secretion. Since this same hypothalamic area controls the release of pituitary ovulating hormone, it is suggested that the neural or neuro-humoral factor(s) stimulating the discharge of gonadotropin inhibit(s) lactation by blocking the release of lactogenic hormone. Once this inhibition is removed lactogenesis occurs.