To evaluate the efficacy of a single oral administration of the new ergot derivative Cabergoline in the prevention of post-partum lactation, we compared the effects of three different doses of the drug with those of placebo in 32 puerperal women. In a controlled, double-blind trial, the subjects were randomly allocated to four treatment groups receiving either placebo or 400, 600, or 800 micrograms Cabergoline (N = 8 in each group) within 24 hours after delivery. Treatment efficacy was assessed clinically by physical examination before (day 0) and at one, two, three, four, and 14 days after treatment. Plasma prolactin (PRL) concentrations were measured in blood samples collected before and at one, two, three, and four days after treatment. Lactation was prevented in four of the eight subjects (50%) who received 400 micrograms Cabergoline and in all subjects who received 600 or 800 micrograms Cabergoline. By contrast, only one of the eight subjects (12.5%) receiving placebo showed no signs of spontaneous lactation within the 14 days after delivery. No effects of placebo administration on plasma PRL levels were observed. Plasma PRL concentrations were significantly reduced starting from one day after Cabergoline administration, however, and the amount of inhibition of PRL secretion induced by different doses of the drug was not statistically different. These preliminary data demonstrate that Cabergoline has a dose-related effect in the prevention of postpartum lactation, and milk secretion can be prevented completely by a single oral administration of 600 or 800 micrograms of the drug.