Ten puerperal women were treated with the gastrokinetic agent Cisapride 20 mg 8-h by oral tablets. Breast milk levels of Cisapride had reached a steady state by treatment day 3. Day 4 breast milk levels were 6.2 +/- 1.3 and serum levels 137 +/- 8.1 ng/ml (mean +/- SEM). The breastfeeding infant of a woman on the above treatment would ingest an amount of Cisapride 600-800 times lower than the usual therapeutic dose.