The interaction of the vasodilator, hydralazine, with the molybdenum hydroxylases, aldehyde oxidase and xanthine oxidase has been investigated. A potent progressive inhibition of rabbit liver aldehyde oxidase, in the presence of substrate, by low concentrations of hydralazine (0.1-1 microM) was observed in vitro but no effect was seen with bovine milk xanthine oxidase. This activity was mirrored in vivo when levels of aldehyde oxidase were significantly decreased in rabbits administered hydralazine (10 mg/kg/day for seven days) whereas hepatic xanthine oxidase activity was unaltered by hydralazine treatment. Various metabolites of hydralazine were synthesized but found to be devoid of in vitro inhibitory activity. Aldehyde oxidase prepared from either guinea pig or baboon liver was inhibited in a similar way to that of rabbit liver.