Relationship between serum ACE activity and mean blood pressure (MAP) after administration of a single oral dose of the ACE inhibitor enalapril 10 and 20 mg tablets was investigated in 19 Iranian normotensive male subjects. Enalapril at doses, which maximally inhibit ACE activity, reduced MAP dose dependently. The t(max) of ACE inhibition decreased significantly by increasing the enalapril doses, but t(max) of MAP reduction did not change by increasing the dose. The AUC (area under the curve) of ACE inhibition versus time was significantly larger in 20 mg enalapril group compare to 10mg enalapril group (p<0.001). A significant correlation was found between log of residual ACE activity and MAP (r=0.4927; p<0.001). It is concluded that in Iranian normal subjects, after administration of a single oral dose of enalapril, MAP related to residual ACE activity.