In a study on enzyme activities in normal human myocardial tissue the activities of lactate dehydrogenase (LDH), alpha-hydroxybutyrate dehydrogenase (HBDH), creatine kinase (CK), and aspartate aminotransferase (AST), and the activities of the isoenzymes of LDH (1--5) and AST (c and m) were measured in myocardial specimens obtained from live patients (18 biopsies) and postmortem (14 autopsies). Comparison of measurements in 4 right ventricular biopsies from 1 patient with those in 9 right ventricular biopsies from 9 patients shows that (1) the enzymes studied are homogeneously distributed throughout the right ventricular myocardium, (2) errors in the measurement of the activities of the enzymes studied are within 5%, and (3) inter-individual differences in myocardial enzyme activities are quite considerable (10--17%). No significant differences in the activities of the enzymes studied were found between left and right ventricular myocardium. The effect of autolysis on the activities of the enzymes studied in myocardial specimens obtained postmortem is small, amounting to -1% to -6% in the first 10 h, irrespective of whether a linear or an exponential decay of enzyme activity is assumed. Comparison of myocardial enzyme activities in biopsies with those in autopsies, the latter being corrected for autolysis-induced inactivation, reveals significant differences with respect to CK, AST and mAST. At present it is impossible to conclude whether these differences were due to the different conditions existing before the myocardial specimens were frozen at -20 degrees C or to the different types of pathology present in the hearts from which the specimens were taken.