The aim of these investigations was to elucidate in which part of the bile system (canaliculi or ductules) antipyrine is secreted actively into bile during extracorporeal perfusion of isolated guinea pig liver. Twelve perfusions were performed including 6 control and 6 with secretin. After 1, 2 and 3 hours from the onset of perfusion the perfusion fluid and bile samples were taken for determination of antipyrine concentration. It was demonstrated that secretin increases the volume of secreted bile, the concentration of antipyrine was decreased and the total amount of antipyrine eliminated with bile remained unchanged. The obtained results refute the hypothesis that antipyrine is secreted into bile in the ductular phase and suggest that this process takes place at the level of biliary canaliculi.