The isotopic dilution method, which permits the in vivo measurements of the rates of the processes involved in cholesterol turnover, has been applied to rats fed a commercial stock diet or a basal semipurified diet in which either the nature and proportions of the source of dietary fiber or the salt mixture were changed. The cholesterolemia was about 100 mg/100 g in rats fed agar-agar, cellulose, bran or the stock diet. Pectin addition (5%) lowered significantly the plasma concentration of cholesterol (70 mg/100 g). Changes in the source of dietary fiber or salt mixture have moderate effects on the absorption coefficient of dietary cholesterol (range 58.2%-82%). In comparison to agar-agar, cellulose at 2.3% in the diet significantly lowered this coefficient, but larger amounts of cellulose (6.8% or 12.3%), or pectin (5%) were without effect, while bran addition (10%) tended to slightly decrease cholesterol absorption. Hence, high levels of cellulose in the diet increased the absorption coefficient in comparison to a low cellulose diet. A decrease of this coefficient was also observed when the calcium content of the diet was increased. Cholesterol biosynthesis and fecal excretion were inversely correlated to the absorption coefficient of dietary cholesterol in rats fed all of the semipurified diets indicating, as previously shown, that the intestine was the major source of biosynthesized cholesterol diverted into the plasma. However, feeding a commercial stock diet greatly increased the cholesterogenesis and the fecal elimination of bile acids, suggesting a high hepatic cholesterogenesis.