Angiotensin-converting enzyme (ACE) is present on the luminal surface of the coronary vessels, mostly on capillary endothelium. ACE is also expressed on coronary smooth muscle cells and on plaque lipid-laden macrophages. Excessive coronary circulation (CC)-ACE activity might be linked to plaque progression. Here we used the biologically inactive ACE substrate ³H-labeled benzoyl-Phe-Ala-Pro ([³H]BPAP) to quantify CC-ACE activity in 10 patients by means of the indicator-dilution technique. The results were compared with atherosclerotic burden determined by coronary angiography. There was a wide range of CC-ACE activity as revealed by percent [³H]BPAP hydrolysis (30-74%). The atherosclerotic extent scores ranged from 0.0 to 66.97, and the plaque area scores ranged from 0 to 80 mm². CC-ACE activity per unit extracellular space (V_max/K_mV_i), an index of metabolically active vascular surface area, was correlated with myocardial blood flow (r = 0.738; P = 0.03) but not with measures of the atherosclerotic burden. These results show that CC-ACE activity can be safely measured in humans and that it is a good marker of the vascular area of the perfused myocardium. It does not, however, reflect epicardial atherosclerotic burden, suggesting that local tissue ACE may be more important in plaque development.