The activity of xanthine dehydrogenase in human postmortem tissues is surprisingly high in brain and heart; activity was found in most tissue samples, whereas many samples contained little or no oxidase activity. We have confirmed the high level of oxidase activity in liver in which tissue conversion of dehydrogenase to oxidase appears complete. We have also confirmed the virtual absence of either activity in fresh human placenta. Fresh rabbit tissues similarly show considerable dehydrogenase activity in brain and heart. In view of the stability and generalised distribution of dehydrogenase activity, our results suggest that some modification of existing ideas on the physiological and pathological roles of the enzyme may be needed.