The effects of feeding 9% beef tallow (BT) or menhaden oil (MO) in a 10% fat-60% sucrose-20% protein diet on renal cortex fatty acid profile, renal lipid peroxide formation potential, and the blood pressure response to a norepinephrine challenge was studied. Male weanling BHE/cdb prediabetic rats were studied after 8 weeks of diet treatment. Half the rats were subjected to a norepinephrine challenge, and their mean arterial blood pressure was determined. Plasma renin and angiotensin II levels were determined in the presence or absence of the challenge. The source of dietary fat had no effect on these measurements. MO fed rats had a greater potential to form lipid free radicals in the kidney than BT fed rats despite the fact that the renal tissue from both groups had an equivalent number of unsaturations on a mole % basis. From these results we conclude that the accelerated renal disease in menhaden oil fed rats is not due to a diet fat effect on blood pressure regulation but might be due to a diet fat effect on free radical production. These free radicals can be cytotoxic and if produced in large amounts could result in a loss of glomerular cells. Whether this occurs and can be reversed by a change in diet was not determined.