It is demonstrated that plasma elimination constants for rapidly eliminated circulating tissue enzymes can be obtained from plasma time-activity curves if a slowly eliminated reference enzyme is simultaneously sampled. Enzyme and reference enzyme must be released together into the plasma. From the elimination constants thus obtained enzyme release into the plasma can be calculated as a function of time. The method can be applied during continuous release of enzyme into the plasma. The validity of the method is tested in the dog by intravascular infusion of a preparation of cytoplasmic enzymes, obtained by incubating dog liver under anoxic conditions. Alanine aminotransferase (ALT) was used as reference enzyme. Infused quantities of aspartate aminotransferase (AST), glucosephosphate isomerase (GPI) and ALT can be estimated with coefficients of variation (CV) of respectively 10, 19 and 7.6%. Application of the method to plasma time-activity curves of enzymes in patients after acute myocardial infarction (AMI), with alpha-hydroxybutyrate dehydrogenase (HBD) as reference enzyme, results in the following values for the fractional catabolic rate constants (FCR) of AST, GPI, creatine kinase (CK) and its isoenzyme CK(MB): FCRAST = 0.093 +/- 0.006 h-1; FCRGPI = 0.27 +/ 0.03 h-1 (mean +/- SE, n = 14); FCRCK = 0.20 +/) 0.02 h-1 (n = 30); FCRCK(MB) = 0.34 +/- 0.08 h-1 (n = 16). These values are considerably higher than mentioned by most authors, and this indicates that enzyme release after AMI has been underestimated. After AMI, enzymes are released in quantities proportional to the enzyme content of human heart tissue. Average release of CK conforms to this rule but large variations for individual patients are observed. Accurate estimates of the quantities of enzymes released into the plasma can be made for slowly eliminated enzymes by the use of fixed mean values for elimination constants. The results presented to this study indicate that tissue enzymes released from infarcted myocardium in patients after AMI are recovered quantitatively in the plasma. Local inactivation of enzymes or inactivation during the transport from heart to plasma is not significant in such patients.