The effects of castration on the incidence of myocardial lesions in rats were investigated in an experiment which included two factors: sex (entire males, castrated males, entire females and castrated (ovariohysterectomized) females) and diets (5% corn oil, 20% corn oil, and 20% Brassica napus var. Zephyr rapeseed oil). For 16 weeks, each of the 12 groups of 30 Sprague-Dawley rats, housed 2 per cage, were fed ad libitum the test oils incorporated in a semisynthetic diet. At each weighing, the mean body weights for each diet were highest in entire males followed by castrated males, castrated females, and entire females with all differences significant (P less than 0.05). The results indicated that castration did not influence cardiac fatty acid composition. The incidence of myocardial lesions in entire and castrated females and in castrated males was similar while significantly more entire males developed lesions (P less than 0.001). Rats fed a diet containing 20% Zephyr rapeseed oil showed a significantly (P less than 0.001) higher incidence of heart lesions than did rats fed diets containing 5% or 20% corn oil. Similarly, significantly (P less than 0.05) more rats fed the 20% corn oil diet had lesions than rats fed the 5% corn oil diet. The involvement of androgens in the formation of myocardial lesions is suggested, since castration significantly lowered the incidence in males but not in females.