Serum activities of creatine kinase (CK, EC 2.7.3.2) and CK isoenzymes, lactate dehydrogenase (LDH, EC 1.1.1.27) and LDH isoenzymes, alpha-hydroxybutyrate dehydrogenase (alpha-HBDH, no EC) and the LDH/alpha-HBDH ratio were studied following a single s.c. application of 5-250 mg isoproterenol/kg body weight (b.w.) in rats. Measurements of the serum enzymes and histological and enzyme-histochemical examinations of hearts were performed 2, 4, 6, 8 and 24 h after treatment. A drastic increase in serum levels of the isoenzymes CK-MB, LDH1, LDH2 and alpha-HBDH and decrease in the ratio LDH/alpha-HBDH were observed from 2 h onwards after isoproterenol application in all dose groups, the maximum effect being after 4-8 h. Focal cellular injury in the myocardium could also be observed from 2 h onwards after isoproterenol application by an enzyme-histochemical method using nitro blue tetrazolium (NBT) whereas the earliest histological alterations using haematoxylin and eosin (HE) stain could be detected 6 h after treatment. A dose-dependent effect as to enzyme values as well as to myocardial necrosis was observed 24 h after isoproterenol application. No kidney damage could be detected on the basis of serum urea nitrogen and histological examinations. Thus, measurement of serum activities of CK-MB isoenzyme alone or LDH1-2 isoenzymes in combination with other tests to exclude kidney damage are valuable indicators of cardiac lesions in rats.