The effects of fumonisin B1 (0, 5, 15 and 45 mg/kg/day), obtained from culture material of Fusarium moniliforme, on drug metabolising enzyme activities were investigated in four groups of five growing ducks by daily oral administration over 12 days. No lethality or sign of toxicosis occurred. The liver and kidney weights were increased, whereas microsomal and cytosolic tissue fractions were unaffected. Although the total microsomal P450 content was unaffected, benzphetamine, ethylmorphine, erythromycin N-demethylases and ethoxyresorufin O-deethylase activities were together increased (respectively by 114, 242, 57 and 27% with 5 mg/kg/day and 1024, 969, 200 and 147% with 45 mg/kg/day). By contrast, aminopyrine and nitrosodimethylamine N-demethylases, methoxyresorufin and pentoxyresorufin O-dealkylases, and UDP-glucuronyltransferase activities were only increased by using 45 mg/kg/day, whereas glutathione S-transferases activities remained unaffected.