Forty-one rats were divided into six groups assigned to one of six diets containing three different fat types (sunflower oil, lard or 50:50 (w/w) blend of sunflower and lard) at one of two levels (5 or 30%) to determine the effect on the corticosterone (Cs) level in the cytosol fraction of the adrenal glands, heart muscle and aorta in rats. Plasma cholesterol concentrations and body composition were also measured. The diet with 5% lard significantly increased adrenal Cs compared with diets with 5% sunflower oil or 5% blend. Marked elevations in Cs level in the aorta were evident in rats fed diets with 5 and 30%o sunflower oil in comparison with rats fed the 5 and 30% blend of lard and sunflower oil. In addition, the Cs level in the aorta in rats fed the diet with 30% sunflower oil was significantly higher than in rats fed the diet with 30% lard. The diet containing 5% sunflower oil markedly elevated the Cs level in rat heart muscle compared with the diet containing a 5% blend of the two fats. Unexpectedly, there were no significant effects of the experimental diets on plasma cholesterol concentrations. Moreover, rats fed diets containing 30% fat had higher body dry matter and fat percentages, but lower protein and ash percentages compared with the rats fed diets containing 5% fat. The percentages of fat, protein and ash in the body were not affected by the type of dietary fat, but rats offered lard diets had a higher percentage of dry matter and fat than rats fed mixed fat diets.