Angiotensin I-converting enzyme (ACE) levels in plasma are stable in a given individual but differ greatly among subjects. To establish the distribution of plasma ACE levels and the mechanisms that are involved in the interindividual variability in this enzyme, plasma ACE has been determined by direct radioimmunoassay in a group of 434 healthy, middle-aged, Caucasian men and compared with several hemodynamic, environmental, and hormonal parameters. The distribution of ACE is assymetric. The interindividual variability is large since ACE levels can differ up to 5.7 times among subjects. No association, however, was observed with candidate environmental or hormonal parameters to explain the interindividual variability in plasma ACE levels. A weak association was observed between ACE and blood pressure. A previous study, conducted in nuclear families, has shown that differences among subjects in plasma ACE levels are, in part, genetically determined. The present study does not reveal other determinants, either hormonal or environmental, of the interindividual variability in this enzyme. The characteristics of the distribution of plasma ACE levels should be considered when a value is interpreted in a given patient.