The effect of typical fat-modified diets on serum lipids, lipoproteins and apolipoproteins, on the intravenous fat-tolerance test and on fecal sterol excretion were observed in a group of men living at home and compared to that of their normal diets. Two 4-week-long diet periods alternated with four periods of 6 weeks' duration during which the men followed their normal dietary habits and accustomed life-styles. According to a randomized cross-over design sunflower oil and corn oil were used as the main fat sources for the fat-modified diets. Both diets lowered total cholesterol considerably acting mainly on LDL cholesterol, without affecting plasma triglycerides significantly. Apolipoprotein A-I and A-II concentrations in serum remained constant, and clearance of triglyceride-rich lipoproteins, as measured by an intravenous fat-tolerance test (Intralipid test), was not significantly affected. While changes of all measured parameters pointed in the same direction, serum total and LDL cholesterol levels were significantly lower with corn oil than with the sunflower oil. Excretion of endogenous sterols (fecal sterol balance) was higher with the fat-modified than with the normal diet. This increase in sterol excretion closely corresponded to the extent of serum cholesterol lowering.