In 14 hypertensives the effect of captopril (angiotensin-converting enzyme inhibitor) treatment on red blood cell Na+, K+, Mg2+ and Zn2+ concentrations and on plasma Zn2+ concentrations were studied. Intraerythrocytic Zn2+ concentrations increased slightly, whereas plasma zinc concentrations decreased. The concurrence of hypozincemia with side effects resembling the symptoms of zinc deficiency such as loss of smell and taste suggests a role of alterations in zinc metabolism for the pharmacodynamics of captopril.