These studies used mice that were deficient in cholesterol 7alpha-hydroxylase to determine the effects of reduced bile acid synthesis on cholesterol homeostasis. In mice lacking this enzyme, bile acid synthesis was reduced from 8.3 to 3.4 micromol/day per 100 g body weight, the intestinal bile acid pool was decreased from 62.5 to 13.2 micromol/100 g bw, and the proportion of hyodeoxycholate, relative to cholate, in this pool was significantly increased. Associated with these changes, intestinal cholesterol absorption decreased from 37% to <1% while triacylglycerol absorption and animal weight gain remained essentially unaffected. The very low rate of cholesterol absorption could be corrected by feeding the mutant mice cholate, but not hyodeoxycholate. The reduction in sterol uptake across the intestine was associated with a 2-fold increase in cholesterol synthesis in the small bowel and liver and an increase in fecal neutral sterol excretion from 15.2 to 35.7 micromol/day per 100 g bw. The size of the cholesterol pools in the plasma, various organs and whole animal remained constant. Thus, under circumstances where the excretion of sterol as bile acids was markedly reduced, total cholesterol turnover actually increased from 164 to 239 mg/day per kg bw. This study demonstrates the complex interactions between bile acid and cholesterol metabolism and the dramatic effects of eliminating a single gene product; however, even though a major catabolic pathway was deleted, cholesterol balance across the animal was maintained.