Studies were carried out on Sprague-Dawley (SD) and Chester Beatty (CB) rats to determine whether the difference in incidence of myocardial lesions can be related to dietary factors and parameters known to be affected in SD rats fed rapeseed oils. The two strains of young, male rats were fed diets which contained 20% by weight of either corn, LEAR (low erucic acid) or HEAR (high erucic acid rapeseed) oils for a period of up to 16 weeks. A significantly lower incidence of focal myocardial necrosis was observed in CB rats than in SD rats. The incidence of this heart lesion in CB rats was similar between all diets; in SD rats a higher incidence was observed in the groups fed rapeseed oils. In both strains the growth rates of rats fed LEAR and corn oils were similar; growth rates with HEAR oil diets were much lower than the other oils. Severe myocardial lipidosis was only evident in rats fed HEAR oil, but no strain differences were observed. The pattern and extent of lipidosis, including cardiac and hepatic lipid levels and fatty acid compositions, were similar in both strains at the time periods studied. Cardiac triglycerides and free fatty acids increased in rats fed HEAR oil, but no strain differences were evident. Strain differences were found in the levels of a few cardiac phospholipids of rats fed HEAR oil, however, the relative fatty acid compositions of each phospholipid were remarkably similar. Microscopic examination of the livers showed no evidence that feeding rapeseed oil caused any specific effect which could be related to differences in heart lesion response. There was no evidence to indicate that differences in heart lesion response between the two rat strains could be related to cardiac triglycerides, free fatty acids or phospholipids.