Frontal plane electrocardiograms (standard limb leads) were recorded on male Sprague-Dawley rats fed for 14 weeks on semisynthetic diets containing 20% (by weight) of Brassica napus cv. Tower or B. campestris cv. Span rapeseed oil or fractions isolated from Span rapeseed oil. Control diets were rat Chow or semisynthetic diets containing 20% (by weight) of either safflower oil or corn oil. The amplitude of the P, Q, R, S, and T waves, the duration of the P wave, QRS complex, and P-R interval, and the cardiac rate were measured in the electrocardiograms and differences between groups compared statistically. While there were statistically significant differences in the amplitude of some of the wave forms, there were no consistent differences which could be attributed to the feeding of rapeseed oil or its fractions. The duration of the P wave and the QRS complex were significantly longer in some of the control groups than those in most of the treated groups. There was no correlation between the presence of a Q wave and the incidence of myocardial lesions in any group. Cardiac rate was essentially the same in all groups.