Desipramine and its metabolite 2-hydroxydesipramine were measured in the milk of a nursing mother and in the plasma of the mother and infant during administration of 300 mg/day of desipramine to the mother. Similar concentrations of both compounds were found in maternal plasma and milk. A pharmacokinetic plot of the milk over 24 hours showed the expected rise and fall of the substances following a single nighttime dose. Neither parent compound nor metabolite could be detected in the infant's serum even though the measurements were made shortly after peak ingestion by the infant. Furthermore, no clinical signs of toxicity were observed in the infant after 3 weeks of treating the mother with desipramine.