Dealkylation of tetraalkyltin compounds (alkyl: ethyl or butyl) in the intestinal mucosa and in the intestinal contents of the rabbits was investigated in situ and in vitro. Tetraethyl and tetrabutyltin compounds were directly infused into the small intestine isolated in vivo by two forceps after making biliary fistula. Amounts of tetraalkyltin compounds and dealkylated compounds, trialkyltin, in portal vein blood, venous blood, bile, and organs were measured as a function of time after the injection. In the earliest stage, the concentrations of dealkylated compounds in portal vein blood were higher than that in systemic venous blood. This result, together with the finding that no conversion occurs in intestinal contents in vitro, suggests that the conversion of tetraalkyltin compounds into trialkyltin compounds takes place in intestinal mucosa before tetraalkyltin compounds are taken into the liver.