Effects of butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), which are widely used as food antioxidants, on plasma disappearance and biliary excretion of organic acid BSP, organic base PAEB or organic neutral compound ouabain were studied. Male Sprague-Dawley rats (200-300 g) were fed the powdered diet containing various doses of BHT and BHA for specified durations (2, 4 or 10 days). After treatment the rat were anesthetized, the common bile duct, femoral artery and vein cannulated, and sulfobromophthalein (BSP), procaineamideethobromide (PAEB) or ouabain administered iv. Plasma concentration of BSP in BHT-treated rats tended to be lower than that of control, whereas biliary excretion of BSP was increased at all doses (0.1, 0.25 and 0.75%) of BHT, especially in first 20 min collection periods. BHA-treatment did not affect on plasma concentration of BSP but significantly increased in biliary excretion. Bile flow and liver wet weight were increased in BHT- and BHA-treated rats. BHT-treatment also significantly increased in biliary excretion of PAEB, especially in the unconjugated free form. Effect of BHT on plasma concentration and biliary excretion of ouabain was different from that of BSP and PAEB. In rats treated with 0.25% BHT plasma concentration was similar to control at 2 and 4 days but was higher at 10 days. Biliary excretion was increased at first 10 min but total amounts ouabain excreted into bile for 60 min was similar to control. This results suggest that the increased biliary excretion of BSP and PAEB may be due to the increased canalicular bile formation by BHT and BHA, and that biliary excretion of ouabain is not affected at the same degree as in BSP and PAEB.