The effects of chronic intake of dietary ethanol upon catalase, an enzyme capable of metabolizing ethanol, as well as upon myocardial morphology and hemodynamics, were studied in the rat. Ethanol, comprising 36% of dietary calories, administered to rats for 5 weeks, was associated with increased myocardial catalase of 45.9 +/- 3.7 IU/mg protein, compared to 21.0 +/- 1.8 IU/mg protein in pair-fed controls. The enzyme activity remained significantly elevated after 18 weeks of ethanol. Hepatic catalase did not differ in these groups. Parallel cytochemical studies confirmed the increase in myocardial catalase by demonstrating an increase in peroxisomes. Gross and light-microscopic examinations revealed no abnormalities at either 5 or 18 weeks. Remarkably few ultrastructural abnormalities were seen in this material fixed by vascular perfusion. Hemodynamic studies after 5 weeks of ethanol revealed decreased left ventricle systolic pressure and decreased mean arterial pressure but no change in ventricular filling pressure. The possibility of catalase playing a metabolic and potentially protective role in rat myocardium chronically exposed to ethanol is discussed.