The elimination of 14C and of 3H was studied in rats after i.v. administration of 0.22--0.33 mg/kg of 14C- or 0.67 mg/kg of 3H-labelled N,N'-dimethyl-N,N'-bis-[3-(3',4',5'-trimethoxy-benzoxy)-propyl]-ethylenediamine-dihydrochloride (hexobendine, Ustimon, Reoxyl), respectively. Regarding the obtained results shortcomings in methods are discussed which may falsify the elimination pattern of radioactively labelled substances. Under certain experimental conditions (closed metabolism chamber with absorption of CO2 or H2O, respectively) 99-100% of the administered radioactivity were detected in the excretion products. The appearance of 14CO2 in the expired air was prevented when the bile duct was cannulated and the bile was drained to the outside before reaching the gut with its probably decarboxylating bacteria.