To assess the disposition of the dihydropyridine calcium antagonist, nitrendipine, in lactating mothers, we studied three breast-feeding women to determine simultaneous plasma and breast milk concentrations of nitrendipine and its inactive pyridine analog metabolite after both a single 10 mg oral dose and 5 days of continuous therapy (20 mg per day). Nitrendipine was excreted in breast milk at peak concentrations ranging from 4.3 to 6.5 ng/ml 1-2 h after acute dosing while its inactive pyridine metabolite ranged from 6.9 to 11.9 ng.ml-1. After 5 days of dosing, Cmax remained in the same range and the breast milk/whole plasma concentration ratio for nitrendipine was 0.2 to 0.5. On the fourth day of continuous dosing, average concentrations of nitrendipine from 24-h collections of the milk were 1.1 to 3.8 ng.ml-1. Thus, nitrendipine and its metabolite are excreted in very low concentrations in human breast milk. Based on a maternal dose of 20 mg daily, a newborn infant would ingest an average of 1.7 micrograms of nitrendipine per day, or a relative dose of 0.095%.