Three groups, each of eight adult male monkeys were maintained for a little over 1 year on diets containing 20% mustard oil, peanut oil or hydrogenated peanut fat, respectively. Animals of the mustard oil group showed sarcoplasmic vacuolation of right and left ventricular myocardium. Half the number of monkeys also exhibited a variable degree of myocardial fibrosis which involved the subepicardial zone of the right ventricular wall between the tricuspid valve and apical region. That these changes were not the manifestations of a high fat intake per se was shown by the absence of such changes in the hearts of the animals consuming similar amounts of peanut oil or hydrogenated peanut fat. The results indicated the possible hazards of consuming diets rich in mustard oil for prolonged periods, which like rapeseed oil contains high amounts of erucic acid.