Weanling male Wistar rats which had been fed for 12 weeks purified diets containing 15% by weight of sunflower oil (SF), high erucic acid rapeseed oil (HEAR) or low erucic acid rapeseed oil (LEAR) and subjected to a treadmill program were compared with sedentary animals fed the same diets, on the basis of cardiac morphology and performances. The rats were trained 5 days a week for 12 weeks, and the speed and duration of the run were progressively increased over 6 weeks until the animals ran continuously for 50 min at 22 m/min with a 4% incline. The following main results were obtained: 1. Food consumption and growth rate decreased in trained animals (Table II). Resting heart rates were not slowed down by training, but were higher in rats fed SF or HEAR than in animals fed LEAR (Fig. 1). Trained animals showed an increasing fatigue during exercise, particularly those fed LEAR (Fig. 2). 2. The ratios of heart weight, muscle weight, kidney weight to body weight, were higher in trained animals, but absolute organ weights were similar in control and trained rats. 3. HEAR caused the highest incidence and number of heart lesions both in sedentary and in trained rats, but in the latter there was a highly significant increase in the number of lesions (Table III). 4. Physical training had different effects on cardiac performances according to the nature of the dietary oil. Thus, the rate of left ventricular pressure rise (dP/dt) decreased only in rats fed, SF or HEAR (Table IV).