The elevation of endogenous prolactin secretion using thyrotropin releasing hormone was associated with significant increases in mammary milk production in postpartum women. Moreover, a specific effect was seen on the percent fat composition which has been shown to rise as much as 228‰ over pretest conditions. As in the bovine, administration of high doses of estrogen is associated with mammary breast development and the sudden removal of this stimulus if accompanied by nipple stimulation is associated with nonpuerperal lactation. The inhibitory effects of estrogen on the mammary cellular response to circulating prolactin has been deduced from studies in pregnant and parturient women by measuring the TRH-induced prolactin response. These studies support a relationship between prolactin and sex steroids on the initiation and maintenance of human lactation. (J Clin Endocrinol Metab40:764, 1975)