Experiments were undertaken to ascertain whether the previously demonstrated inhibition of the development of experimental aortic atherosclerosis in alloxan-diabetic rabbits fed cholesterol was due to the injection of alloxan per se or to the existence of the diabetic state produced by alloxan. It was established that, by treating the diabetic state with insulin, the diabetic state could be ameliorated and the inhibitory effect obviated. It was therefore concluded that the inhibitory phenomenon was not due to the injection of alloxan per se but that it was associated with one or more factors that characterize the alloxan diabetic state in the rabbit and that are reversible by insulin therapy. In the course of the experiment it was demonstrated that the inhibitory effect was apparent in cholesterol-fed diabetic rabbits whether or not their diet was supplemented with vegetable oil. The previously reported metabolic abnormalities of the diabetic animals were confirmed. It was established that suitable treatment of the cholesterol-fed diabetic animals with insulin would bring all the metabolic aberrations, including those of the serum lipids, into reasonably close correspondence with those observed in non-diabetic rabbits fed cholesterol.