Forty-two primiparous and 54 multiparous women with total yields of milk not exceeding 50 ml for the first 3 postpartum days were treated with 100 mg of oral sulpiride or placebo for 4 days from the third postpartum day. In the primiparous mothers, the mean daily yield of milk in the sulpiride group increased significantly (p less than 0.01) over that in the control group after the fourth postpartum day. Mean (+/- SE) total volumes of milk for the third to fifth postpartum days were 661.5 +/- 64.4 and 441.2 +/- 51.2 ml in the sulpiride and the control groups, respectively. However, in the multiparous mothers, no significant difference between the control and the sulpiride group was noted in total yield of milk, since a good increase in the secretion of milk was obtained without medication in these mothers. Determinations of daily serum levels of prolactin in 20 primiparous women revealed significantly higher concentrations in the sulpiride group. The ratio of primiparous mothers with complete breast-feeding 1 month after delivery was higher in the sulpiride group (55%) than in the control group (30%), whereas there was no difference between the control group and the sulpiride group of multiparous women. These data indicate that poor initiation of puerperal lactation in primiparous mothers can be effectively treated with oral sulpiride.