A double-blind trial was performed on 26 women, who had elected not to breast-feed their infants, to determine the effect of bromocriptine mesylate (2.5 mg twice daily for 14 days postpartum) on the composition of the mammary secretion during lactogenesis. Mammary secretion (less than 5.0 ml) was collected from each breast of each woman at daily intervals during the 14-day treatment period and the progressive changes in the concentration of the milk constituents, lactose, alpha-lactalbumin, serum albumin, total protein, lactoferin, IgA, IgG, sodium and potassium were determined. The degree of milk leakage and breast engorgement were also assessed. Both the subjective assessments and the changes in the concentration of the milk constituents demonstrated that lactogenesis occurred between about Day 2 and Day 5 postpartum in the placebo group (in the absence of the suckling stimulus) but was suppressed in the bromocriptine-treated group.