The angiotensin I-converting enzyme (kininase II, ACE) is the major angiotensin II forming and kinin degrading enzyme in the circulation, and has other physiological peptide substrates. Recent studies have established that the interindividual variability in the levels of ACE in plasma and tissues is under the influence of a genetic polymorphism. The genetic polymorphism of ACE levels has been linked in case-control studies to the susceptibility of developing cardiovascular diseases, especially myocardial infarction and diabetic nephropathy, and to the risk of progression of renal diseases. The new concept that the level of ACE in peripheral circulations and tissue interstitium is an important factor in the determinism of the local concentration of peptides and their putative protective/deleterious effects, especially in the kidney and the heart, will be further appraised.