ME 2303 is a new anthracycline analog which differs from adriamycin in the sugar moiety. ME 2303 displays a higher antitumor activity against L 1210 and P 388 Leukemia than adriamycin. To evaluate the cardiotoxicity of ME 2303, ECG tracings (21 patients) and Holter ECGs (9 patients) were recorded before and after the administration of ME 2303 for various malignancies (mean dose 123.8 mg/m2), and some of the electrocardiographic parameters were analyzed. Control ECGs were normal in 17 patients, in 4 patients minor ECG findings like sinus tachycardia (2 patients), low voltage (1 patient) and flattening of the T wave (1 patient) were observed. After treatment, no relevant ECG changes were observed except one case who showed a flat T in pretreatment ECG. In this patient developed ST-T changes were seen after treatment. The basic rhythm was sinus rhythm in all of the cases, and the heart rate showed no significant changes. ME 2303 had no effects on the specialized conduction system. With regard to arrhythmia, no increase in number and severity was observed. In Holter ECG no development of ST-T changes was seen after treatment.