Cytochrome P450-associated monooxygenase activities were measured in control and prototype inducer-treated mallard duck, black duck, wood duck, lesser scaup, Canada goose and mute swan. Ages of the birds ranged from pipping embryos (that were treated approximately 3 days before pipping) to adults. Three or more of the following hepatic microsomal monooxygenases were assayed in each species: Benzyloxyresorufin-O-dealkylase (BROD), Ethoxyresorufin-O-dealkylase (EROD), methoxyresorufin-O-dealkylase (MROD), and pentoxyresorufin-O-dealkylase (PROD). Baseline activities differed between species, but because of differences in ages, sources of the eggs or birds, and diets, these cannot be viewed as absolute differences. The cytochrome P450 inducers utilized were beta-naphthoflavone (BNF), 3-methylcholanthrene (3MC) and phenobarbital (PB). In general, there was little response to PB; only lesser scaup were induced to greater than three times control level and most species were well under this. Responses to BNF and 3MC occurred in each species studied, but differed in which of the monooxygenases was most induced (absolute values and ratios to control values) and in relative induction between species. BROD frequently had an induction ratio EROD. Overall, lesser scaup were the most responsive, canada geese the least responsive, and the other species intermediate in responsiveness to the cytochrome P450 inducers studied.