The possible effect on the infant of dopamine antagonists used to promote lactation is cause for concern. Domperidone (Motilium) may be safer than other drugs in this group as it does not cross the blood-brain barrier. The mean serum level of prolactin 2 h after treatment with 20 mg of domperidone in the puerperium was 255 ng/ml compared with 150 ng/ml after a placebo. The mean domperidone level in all breast milk samples during treatment with 10 mg, three times daily, was 2.6 ng/ml. This was significantly more than levels after a single 20 mg dose sampled at 2 h (0.24 ng/ml) and at 4 h (1.1 ng/ml), and considerably less than values available for metoclopramide and sulpiride, relative to the therapeutic dosage. The effectiveness of domperidone to augment lactation requires further study.