The effects of "long-term" feeding of different lipid supplemented diets (12% added fat, w/w) on the incidence of lipidosis or the severity of necrosis was examined in the cardiac muscle of male Hooded Wistar rats, after at least 12 months on the diets. The effects of supplementation with either n-6 polyunsaturated fatty acids (PUFA's) added as sunflower seed oil (the SSO diet), or one enriched with n-3 PUFA's added as a low cholesterol, low vitamin (A & D) fish oil preparation obtained from eviscerated Southern Bluefin Tuna (the TFO diet) were compared to those found in the hearts of rats fed either a relatively low fat commercially available stock diet (REF) which contained 4% (w/w) of mixed fats of animal, vegetable and marine origin, or after this stock diet had been supplemented to the same extent by the addition of 12% (i.e. 12:88 g) sheep kidney (perirenal) fat, the SF diet. Extensive cardiac lipidosis was seen after feeding either the TFO or the SF diets, but was not observed in the hearts of experimental rats from either the SSO or REF fed groups. Conversely, in these mature animals, grade 1 necrotic lesions were uniformally found in the cardiac muscles of all rats examined, but neither their incidence nor severity could be attributed to any dietary effect. These necrotic lesions are therefore more probably a reflection of the age of the animals at the time of sacrifice, rather than to any of the dietary supplements employed. Some evidence of "Yellow Fat" disease was found by the presence of lipofuscin pigmentation in the storage fat of rats receiving n-3 PUFA's (the TFO diet) but was not observed in any other dietary group nor in the livers or kidneys of any animals. This extent of storage fat pigmentation was not associated with any retardation of growth in this dietary group.