Benzydamine (BZ), a weak base and an indazole derivative with analgesic and antipyretic properties used in human and veterinary medicine, is metabolized in human, rat, cattle and rabbit to a wide range of metabolites. One of the main metabolites, BZ N-oxide (BZ-NO), is produced in the liver and brain by flavin-containing monooxygenases (FMOs), by liver and brain enzymes. To evaluate the suitability of BZ as an FMO probe in veterinary species, BZ metabolism was studied in vitro using liver microsomes from bovine, rabbit and swine. Kinetic parameters, K(m) and V(max), of BZ-NO production, were evaluated to corroborate the pivotal role of FMOs. Inhibition studies were carried out by heat inactivation and by specific FMO chemical inhibitors: trimethylamine and methimazole. The results confirmed the presence of FMO activity in the liver and the role of BZ as a suitable marker of FMO enzyme activities for the veterinary species considered.