Blood and milk concentrations of captopril, a new antihypertensive agent, were studied in 12 lactating normotensive subjects after the administration of 100 mg captopril tablets on a three times daily regimen for a total of seven doses. Peak blood concentrations (+/- S.E.M.) of captopril after the administration of the seventh 100-mg tablet averaged 713.1 +/- 140.6 ng/ml, as compared to average peak milk concentrations of 4.7 +/- 0.7 ng/ml. Time to peak blood concentrations averaged 1.1 +/- 0.2 hour, while time to peak milk concentrations averaged 3.8 +/- 0.6 hours. The data suggest that the human breast selectively restricts the passage of captopril from blood into milk.