The mechanism by which female rats maintain a higher level of serum cholesterol than males on cholesterol containing diets was studied. Retention of neutral steroids and excretion of bile acids were significantly higher in females than in males. The composition of fecal bile acids was also sex-dependent. Lithocholic and α-muricholic acids were predominant in females and hyodeoxycholic in males. It was concluded that the sex differences in blood cholesterol levels are mainly due to the higher retention rate of neutral steroids by the females and not by their limited capacity to excrete bile acids