The distribution of relative aldehyde dehydrogenase activity using acetaldehyde, propionaldehyde and benzaldehyde as substrates in various nuclei of rat brain was determined using the quantitative histochemical method. ALDH activity was higher in a majority of the nuclei with benzaldehyde, rather than acetaldehyde or propionaldehyde as a substrnte. In brain microregions the highest enzyme activity was detected in ependyma, suggesting that ALDH at the blood-brain barrier may play an important role in exogenous and/or endogenous aldehydes metabolism. Nucleus of mesencephalic hart of the trigeminal nerve, motor nuclei of cranial nerves of the brain stem, and arcuate hypothalamic nucleus showed much higher ALDH activity with benzaldehyde and while the inferior colliculus, the reticular part of substantia nigra, and nuclei of the lateral lemniscus the lowest activity. The proportion of neurons with ALDH activity was different according to different nuclei. These results suggest that in functional aspect, ALDH activity may be more closely related to motor function than sensory function in the central nervous system, and that nerve cells in individual nuclei may have an aldehyde metabolic difference.