The ABC half-transporters ABCG5 and ABCG8 heterodimerize into a functional complex that mediates hepatobiliary sterol secretion. We have addressed the putative rate-controlling role of Abcg5/Abcg8 in mice during stimulation of this process. Despite similar bile salt (BS) excretion rates, basal total sterol and phospholipid (PL) output was reduced by 82 % and 35 %, respectively, in Abcg5^-/- mice. Upon infusion with the hydrophilic bile salt TUDCA, similar relative increases in bile flow, BS output, PL output and total sterol output were observed in wild-type, Abcg5^+/- and Abcg5^-/- mice. Maximal cholesterol and PL output rates in Abcg5^-/- mice were only 15 % and 69 %, respectively, of wild-type values. Infusion of increasing amounts of the hydrophobic bile salt TDCA increased cholesterol excretion 3.0- and 2.4-fold in wild-type and Abcg5^-/- mice, but rapidly induced cholestasis in Abcg5^-/- mice. Treatment with the LXR agonist T0901317 increased the maximal sterol excretion capacity in wild-type mice (4-fold), concomitant with induction of Abcg5/Abcg8 expression, but not in Abcg5^-/- mice. In a separate study, mice were fed chow containing 1% cholesterol. As expected, hepatic expression of Abcg5/Abcg8 was strongly induced (4-fold) in wild-type but not in liver x-receptor alpha-deficient (Lxra^-/-) mice. Surprisingly, hepatobiliary cholesterol excretion was increased to the same extent, i.e., 2.2-fold in wild-type and 2.0-fold in Lxra^-/- mice, upon cholesterol-feeding. Our data confirm that Abcg5, as part of the Abcg5/Abcg8 heterodimer, strongly controls hepatobiliary cholesterol secretion in mice. However, we demonstrate that Abcg5/Abcg8-independent, inducible routes exist that can significantly contribute to total hepatobiliary cholesterol output.