Quantitative assays of alcohol dehydrogenase and its endogenous inhibitor in several tissues at different stages of ontogenesis have been performed. The results show that the levels of both enzyme and inhibitor activities fluctuate during the course of differentiation in maize plants. The inhibitor is probably not a phenolic substance, as indicated by the fact that it is unaffected by PVP in the medium and shows varying effectiveness against alcohol dehydrogenases from different organisms. During kernel development (from the 15th day after pollination to the dry seed) the specific activity of alcohol dehydrogenase in maize scutellum remains high, and subsequently declines sharply after the onset of germination. The rapid and selective elimination of alcohol dehydrogenase in various maize tissues during development seems to be mediated by the endogenous ADH inhibitor. Density-labelling experiments show that very little alcohol dehydrogenase is synthesized de novo during early germination under normal conditions, but that de novo synthesis can be induced by subjecting the seedlings to anaerobiosis.