Plasma disappearance of amaranth (AM), a model compound of organic anionic drugs, was retarded by intravenous infusion of taurodeoxycholate (TDC), a representative bile acid, in the rat. Biliary excretion accounted for 30-60% of the systemic excretion of AM. AM seemed to be metabolised in the hepatocyte to form a compound that is excreted more rapidly into the bile than AM itself, considering apparent biliary clearance, $CL_{bil}$, is much larger than systemic clearance, $CL_s$. Decrease in $CL_{bil}$ by TDC infusion might be due to elevated plasma level rather than decreased biliary excretion of AM. Decreased distribution or urinary excretion of AM by TDC was supposed to be one of the probable reasons of elevated plasma level. Competitive inhibition between AM and TDC on tissue distribution and urinary excretion might explain the mechanism. The effect of TDC on the $CL_{bil}$ of methylene blue, a cationic dye, was quite different from that of AM, as reported previously by us. More intensive study would be necessary to elucidate the difference of biliary excretion between organic anions and cations.