Thiourea, phenylthiourea, and methimazole perfused into rat liver stimulated the biliary efflux of GSSG without affecting the excretion of GSH into either the bile or the caval perfusate. The thiocarbamide moiety appears essential, since perfusion with urea, phenylurea, or N-methylimidazole did not stimulate GSSG release. Hydrogen peroxide is also not an obligatory intermediate, since thiocarbamide-induced GSSG efflux was undiminished in livers from selenium-deficient animals. The response was also not affected by N-benzylimidazole, a potent cytochrome P-450 inhibitor, which suggests that this monooxygenase is not involved. However, the results are consistent with a model based on S-oxygenation of thiocarbamides to formamadine sulfenates catalyzed exclusively by the flavin-containing monooxygenase. The resulting sulfenate is reduced by GSH, yielding GSSG and the parent thiocarbamide. Rapid cellular oxidation of GSH by this mechanism leads to biliary efflux of the disulfide.