ACE inhibitor drugs decrease mortality by up to one-fifth in cardiovascular patients. Surprisingly, there are reports dating back to 1979 suggesting the existence of endogenous ACE inhibitors. Here we investigated the clinical significance of this potential endogenous ACE inhibition. ACE concentration and activity was measured in patient's serum samples (n = 151). ACE concentration was found to be in a wide range (47-288 ng/mL). ACE activity decreased with the increasing concentration of the serum albumin (HSA): ACE activity was 56 ± 1 U/L in the presence of 2.4 ± 0.3 mg/mL HSA, compared to 39 ± 1 U/L in the presence of 12 ± 1 mg/mL HSA (values are mean ± SEM). Effects of the differences in ACE concentration were suppressed in human sera: patients with ACE DD genotype exhibited a 64% higher serum ACE concentration (range, 74-288 ng/mL, median, 155.2 ng/mL, n = 52) compared to patients with II genotype (range, 47-194 ng/mL, median, 94.5 ng/mL, n = 28) while the difference in ACE activities was only 32% (range, 27.3-59.8 U/L, median, 43.11 U/L, and range 15.6-55.4 U/L, median, 32.74 U/L, respectively) in the presence of 12 ± 1 mg/mL HSA. No correlations were found between serum ACE concentration (or genotype) and cardiovascular diseases, in accordance with the proposed suppressed physiological ACE activities by HSA (concentration in the sera of these patients: 48.5 ± 0.5 mg/mL) or other endogenous inhibitors. Main implications are that (1) physiological ACE activity can be stabilized at a low level by endogenous ACE inhibitors, such as HSA; (2) angiotensin II elimination may have a significant role in angiotensin II related pathologies.