Furazolidone (50 mg/kg for 5 days) given orally to rabbits produced a significant inhibition of diamine oxidase (DAO) activity in various tissues. 2. Suppression of the rabbits' gut flora by oxytetracycline treatment significantly reduced the DAO inhibitory effect of furazolidone, suggesting that the gut flora may be involved in the transformation of furazolidone into an active DAO inhibitor.