Chick embryos were treated on day 0 of incubation with two phenoxy herbicides, 2-methyl-4-chlorophenoxyacetic acid (MCPA) (0.4, 2 mg/egg) and 2,4-dichlorophenoxyacetic acid (2, 4-D) (1, 2, 4 mg/egg). Both herbicides seemed to exert toxic effects mainly on the liver of 19-day-old embryos. Specific histological analysis indicated biliary stasis. Ethoxycoumarin O-deethylase was depressed by MCPA but raised by 2, 4-D. Other hepatic monooxygenase activities were unaffected by the herbicides and no significant changes were found in cytochromes. The higher dose of MCPA increased NADPH-cytochrome P450 reductase. 2,4-D treatment increased by activity of glutathione-S-transferases in the hepatic post-microsomal fraction while MCPA increased them at the lower dose and significantly reduced them at the higher. The phenoxyacetic herbicides appear thus to have some effects on hepatic drug metabolizing enzymes of the chick embryo which cannot be easily interpreted. Biliary retention, produced in particular by MCPA, could be partly responsible for these effects.