A series of studies of cholesterol metabolism in the Cebus monkey were carried out in an attempt to understand the mechanisms responsible for the great differences in serum cholesterol levels when different dietary fats were used. Three groups of monkeys, one fed diets including 45 per cent of calories as corn oil, a second corn oil plus cholesterol (0.1 gm./100 calories), and a third lard plus cholesterol for 5 months (mean serum cholesterol values were 237, 268, and 601 mg. per cent, respectively) were injected with emulsions of cholesterol-4-C(14). The mean biological half-lives for the disappearance of serum radiocholesterol were 8.8, 8.4, and 6.6 days respectively. Esterification of radiocholesterol as measured by equilibration of specific activities of serum-free cholesterol and total cholesterol was delayed in the monkeys fed lard plus cholesterol. When cholesterol-4-C-(14)-stearate was given intravenously to a series of monkeys, an erratic non-exponential biological decay curve resulted. Specific activity for free serum cholesterol was greater than that for total cholesterol within 1 hour after the injection. After 7 months on experimental diets including corn oil with added cholesterol and lard with added cholesterol the levels of lipides in most tissues were not different for the two dietary groups, nor were they appreciably elevated above previous control figures for monkeys not fed cholesterol. Total lipide levels in the adrenals of monkeys fed corn oil were twice those of monkeys fed lard. Monkeys were fasted before and after intragastric administration of cholesterol-4-C(14) in small formula meals including various fats and fatty acids. The disappearance of total cholesterol from the serum consisted of a rapid followed by a slow exponential function. The type of fat and fatty acid appeared to influence the rate of disappearance of radiocholesterol. There was a broad range of apparent activity of the different fats and fatty acids in promoting cholesterol absorption.