A new strain of genetically obese rat recently obtained in our laboratory exhibits endogenous hyperlipidemia (marked hypertriglyceridemia and moderate hypercholesterolemia) and spontaneous hypertension. The animals die prematurely from kidney failure or from the complications of atherosclerosis. A low calorie diet proved to be highly beneficial to these rats. Body weight declined, obesity diminished, the hypertriglyceridemia was almost eliminated, and the hypercholesterolemia was reduced. However, the hypertensive state was not alleviated. Since the life span of the rats was greatly prolonged by a low calorie diet, the latter undoubtedly served to prevent or arrest the development of renal and vascular disease in these obese animals.