Increasing doses of clonidine enhanced the retention of sulfobromophthalein (BSP) in plasma and liver, while reducing elimination of this dye into bile. The ED50 of clonidine for these effects was 0.05 to 0.2 mg/kg s.c. In clonidine-treated mice which were warmed to reverse drug-induced hypothermia, plasma and liver BSP levels were raised as compared to saline-treated mice. Clonidine also raised plasma and liver levels of the BSP analog, dibromosulfophthalein, which is not conjugated before biliary elimination. Hepatic glutathione levels, activity of glutathione-S-transferase and ratios of conjugated to unconjugated BSP were not affected by clonidine. In mice with cannulas in their common bile ducts to prevent duct spasm, clonidine reduced the amounts of BSP eliminated into bile. Thus, the alpha-2 adrenoceptor agonist, clonidine, raised plasma and liver levels of anionic dyes and reduced their levels in bile by mechanisms other than altered conjugation, hypothermia or bile duct spasm.