Male rats were fed for 3–4 months (short-term) or 12–15 months (long-term) on a standard laboratory diet alone (control) or supplemented with sunflower seed oil (SSO, 12 % w/w) or sheep kidney fat (SKF, 12% w/w). Papillary muscles were electrically driven (1 Hz, 5 ms, supramaximal voltage) at 37 °C in Krebs-Henseleit solution, and contractions were measured isometrically. Both the positive inotropic responses to CA++ and the incidence of spontaneous tachyarrhythmias under catecholamine stress were increased by short-term SKF feeding and with age in control and SKF groups, whereas SSO prevented these changes. The results show a marked effect of age upon ventricular myocardial function in the rat, which appears to be accelerated by the consumption of animal (saturated) fat while polyunsaturated vegetable oil provides some degree of protection. It is suggested that changes in membrane lipid composition can alter the Ca++ handling characteristics of myocardial c