The present study was designed to investigate and compare the effects of dietary antioxidants on the mechanical characteristics of the rat heart. Both sex weanling rats were fed for 12 to 14 weeks a standardized selenium (Se)- and vitamin E-deficient diet, a Se-excess diet, or a control diet. Deficiency or toxicity of Se was verified by direct (tissue Se analysis and histopathological investigations) methods. The hearts of both experimental groups revealed some alterations in contractile performance with increased heart rate and coronary perfusion pressure. The average peak contractile force of the electrically stimulated papillary muscle measured in both experimental groups was not significantly different from the control values. When expressed as a percentage, the maximal increase in the peak contractile force of papillary muscle (PCF) that was obtained with 100 nM isoproterenol, respectively, was less in both experimental groups (26% in PCF of deficient group; 34% in PCF of rich group) than in the control group (80% in PCF). A decreased stimulation of contractile force of papillary muscle strips by a β-adrenergic agonist seems to be in agreement with possible alterations in the response to inotropic agents due to a modification of the receptor function.