The di-acid metabolite of enalapril, enalaprilat, and its lysine analogue lisinopril are potent inhibitors of angiotensin converting enzyme (ACE); they do not contain sulphydryl groups. Both drugs can be assayed by high pressure liquid chromatography and by radioimmunoassay and plasma ACE inhibition remains stable under normal storage conditions. It is therefore possible to study their pharmacokinetics as well as their pharmacodynamic effects in man. Enalaprilat and lisinopril as well as ACE activity have been measured in blood taken during the course of two studies of the effects of these drugs on blood pressure and autonomic responsiveness. A population pharmacokinetic analysis approach applied to a few concentration-time data points in each of a relatively large number of subjects provided average population parameter estimates of the absorption rate constant, volume of distribution and clearance which correspond closely with the limited published data based on conventional pharmacokinetic approaches. It also provided estimates of pharmacodynamic parameters and the concentration of the drug required to produce a 50% ACE inhibition. Population drug concentration data obtained in the course of early clinical evaluations of new drugs may provide a rational basis for dosage regimens with improved efficacy and, in particular, reduced concentration-related toxic effects.