Rats bearing a biliary fistula received i.v. a solution of Triton WR 1339. Bile and plasma lipid composition was studied every 2 hrs and compared to that of control rats injected with saline. Two hours after Triton injection a sharp decrease in the bile secretion of lecithins (-- 93%), cholesterol (-- 50%) and bile salts (-- 50%) was observed together with a fall in bile flow (-- 20%). Eight hours after Triton administration the biliary output of lecithins, cholesterol and bile salts was lowered to --73%, --50% and --34% respectively compared to control animals. At that time an accumulation of L.C.A.T. substrates (lecithins and cholesterol) was observed in the plasma of Triton group. These modifications of biliary lipids after inhibition of L.C.A.T. activity by Triton W 1339 could be the result of a decreased production of plasma lysolecithins and cholesterol esters suggesting that both lipids could be important precursors for the synthesis of bile constituents. Furthermore this support the view that the production by the liver of plasma and bile lipids follows two distinct pathways.