Recent investigations have shown that cardiac isoenzymes change with mechanical overload and possibly with myocardial ischaemia. This complicates the interpretation of serum enzyme changes in acute myocardial infarction. We have therefore investigated the rate of release of isoenzymes from necrosing myocardium and the effect of ischaemia per se. Discrete myocardial infarction was produced in 35 male Wistar rats by ligation of left coronary artery. Six (n = 7), 12 (n = 6), 24 (n = 9), 72 (n = 7) h and 3 weeks (n = 6) after surgery, total and isoenzyme activities of creatine kinase (CK), lactate dehydrogenase (LD) and aspartate aminotransferase (AST) were measured in the infarcted myocardium. Untreated rats (n = 12) were used as the control (time 0). Sham operation was performed in 36 rats. During the early period (0 to 12 or 24 h) of infarction, each (iso)enzyme disappeared monoexponentially from the myocardium (mean r = 0.88) with different disappearance rates. Cytosolic isoenzyme fractions decreased more rapidly than mitochondrial fractions. CK MB and the LD-H subunit decreased faster than CK MM and the LD-M subunit. Such differences in the disappearance rate may be related to subcellular localisation of each isoenzyme. In the late period (72 h and 3 weeks), CK BB and the LD-M subunit showed significant reaccumulation in the infarcted myocardium. Although inflammatory cells can be responsible for the reaccumulation of LD-M subunit, the origin of CK BB is unknown.(ABSTRACT TRUNCATED AT 250 WORDS)