The hepatic lobular localization of alcohol dehydrogenase (ADH) activity was examined histochemically in mice, rats, hamsters and guinea-pigs with the nitro blue tetrazolium method. In the mouse liver, the ADH activity was localized mainly in the centrilobular zone. In the rat and hamster livers, it was observed throughout the lobule, with increased centrilobular staining. Interestingly, in the guinea-pig liver, if was found to be evenly localized in both the centrilobular and periportal zones, with little or no midlobular staining. These findings show that the distribution pattern of ADH activity in the hepatic lobule of the guinea-pigs differs strikingly from that of the mice, rats and hamsters.