Psyllium (PSY), a type of dietary fiber containing mainly soluble components, has been shown to decrease serum cholesterol concentrations in several species; however, mechanisms involved are not clearly defined. Four groups of 10 rats were fed semipurified diets containing 10% dietary fiber from cellulose and/or PSY for 21 d. Increasing levels of PSY were fed (0,3.33, 6.67 and 10% PSY) with the remaining 10% made up with cellulose. Liver cholesterol, cholesterol 7alpha-hydroxylase (CYP7A) activity and mRNA, 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) mRNA, ileal apical sodium-dependent bile acid transporter (ASBT) mRNA, fecal bile acids and total steroids, and intestinal bile acid content were measured. All variables responded in a dose-dependent manner to PSY in the diet. Total liver cholesterol content was significantly reduced in all groups fed PSY compared to cellulose-fed controls [138(a), 105(b), 105(b) and 93(c) micromol (SEM = 4.2) for 0, 3.33, 6.67 and 10% PSY, respectively]. Activity of CYP7A was significantly greater in all groups fed PSY compared to the cellulose-fed controls [6.36(c), 16.92(b), 15.28(b) and 20.37(a) pmol x min(-1) x mg protein(-1) (SEM = 3.19) for 0, 3.33, 6.67 and 10% PSY, respectively]. These differences in CYP7A activity were similar to differences in CYP7A, HMGR and ASBT mRNA levels. Fecal bile acid and total steroid excretion as well as total intestinal bile acids were significantly greater in rats fed PSY-containing diets compared to 0% PSY-fed rats. These results suggest that the reduction in liver cholesterol involves modulating the size and composition of the bile acid pool via regulation of ileal ASBT, CYP7A and HMGR mRNA levels.