The direct effects of 100, 300 and 500 mg/100 ml ethanol on contractility of isolated, supported right ventricular papillary muscles were evaluated from 10 normal or failing cat hearts. In isometrically contracting muscles, 500 mg% ethanol decreased maximum tension in normal and failing hearts from 7.1 to 4.3 g/mm2 (P less than 0.001) and 3.4 to 2.3 (P less than 0.01) respectively, and lowered maximum tension rise from 30.4 to 20.2 g/mm2/sec (P less than 0.001) and 10.2 to 0.8 (P less than 0.01), without alterations of time to peak tension. In isotonically contracting muscles, 500 mg% ethanol reduced contractile element velocity at 0.5 g/mm2 load in normal and failing ventricles from 1.27 to 0.97 L/sec (P less than 0.001) and 0.59 to 0.39 (P less than 0.001) respectively. Thus, clinically meaningful doses of ethanol clearly, exerted dose-related negative inotropic actions on both normal and failing myocardium, thereby indicating that ethanol ingestion may exacerbate heart failure in diseased hearts.