Adriamycin, an anthracycline antibiotic used for the treartment of tumors, often causes the development of cardiomyopathy. To study initial alterations of myocardial function and ultrastructure, we injected adriamycin to rats in single doses 0,44, 1,31 or 2,19 mg/kg that were similar to therapeutic doses. Even at lowest dose of adriamycin alterations occurred in 5% of cardiomyocytes; myofibrillar alterations appeared as local lysis or contracture, while membrane alterations - as dilation of perinuclear space, sarcoplasmic reticulum and T-system, swelling of mitochondria. At higher doses membrane alterations prevailed. They were observed in 43 and 52% of cells at 1,3 and 2,2 mg/kg, respectively, while myofibrillar alterations were seen only in 15%, of cells. Addition of adriamycin to perfusion solution in concentrations 30 and 50 mu m which were close to two higher doses of adriamycin injected in vivo was associated with decrease of left ventricular developed pressure (by 41 - 44%) and increase of diastolic pressure in the isolated isovolumic heart. The diastolic pressure which in isovolumic mode reflected changes of myocardial diastolic tension was significantly higher in hearts with increased myocardial edema as estimated by the ratio of wet to dry weight of the heart. The results showed a correlation between extent of membrane alterations and negative inotropic effect of adriamycin.