The pathological effects of dietary rapeseed oil (RSO) in rats have been investigated. Rats given 60 cal% RSO for 2 weeks showed fatty accumulation in the heart, skeletal muscles and adrenals. In another experiment, 60 cal% RSO was given for 2, 4, 8 and 16 weeks. The fatty infiltration of the skeletal muscles, adrenals and hearts regressed with the increase in the dietary period, but in the heart, necrotic foci, aggregations of mono-nuclear cells, and an increase in the connective tissue elements ensued. From a 2-week dose-response study, 20 cal% RSO was established as the minimum level causing the fatty accumulation. Similar lesions were produced by feeding RSO or glyceryl trierucate on an isocaloric erucic acid basis. Erucic acid thus appears to be the constituent in RSO responsible for its pathological effects. It was shown that the fatty infiltration of the heart and skeletal muscles occurs after 1 day of RSO feeding and becomes most severe after 3–6 days. The myocardial fatty infiltration in rats given RSO for 3 days markedly decreased after the feeding of this oil had been discontinued. It is suggested that the fatty infiltration of the heart and skeletal muscles by RSO reflects a slower oxidation of fatty acids in these organs.