Thirty-six puerperal women with inadequate lactation were treated with 50 mg sulpiride (N = 24) or placebo (N = 12) three times a day for two weeks. These treatments were supplemented with buccal oxytocin (100 IU, 300 IU, or 400 IU) or placebo preceding each breast-feeding on the sixth and 14th and on the seventh and 13th days of oral treatment, respectively. One woman treated with sulpiride and three women treated with placebo discontinued the trial because of the lack of the effect of treatment. The concentration of prolactin in maternal serum was higher (P less than .001) during sulpiride than placebo treatment at one week (380 +/- 43 ng/ml vs 23 +/- 7 ng/ml, mean +/- SE) and two weeks of treatment (381 +/- 38 ng/ml vs 34 +/- 10 ng/ml). Also, the daily breast milk yield, as measured objectively, was greater (P less than .05) during sulpiride than placebo treatment both at one week (628 +/- 51 ml vs 440 +/- 68 ml) and two weeks of treatment (684 +/- 67 ml vs 423 +/- 60 ml). Various doses of oxytocin failed to stimulate the milk secretion in the presence or absence of sulpiride-induced hyperprolactinemia. Thus, sulpiride improved inadequate lactation, whereas exogenous oxytocin alone or together with sulpiride had no effect on lactation in these mothers whose infants were able to suckle normally.