It was found that the accumulation of acetaldehyde produced from 50 mM ethanol in rat brain homogenates takes place in all major brain regions. The velocity varied between 3.5 to 7.1 nmol/mg of protein/hr. The rate increased in the following order: brain hemispheres, striatum, brainstem, hypothalamus, and cerebellum. Significant regional differences in this process were found: in the initial period of incubation (5 min), acetaldehyde accumulation was maximal in the brain hemispheres; but, in the 30- to 60-min period, it became significantly higher in the cerebellum. Inhibition of this process by the catalase inhibitor, 3-amino-1,2,4-triazole (8 mM), was minimal in the brainstem (27%) and maximal (57%) in the cerebellum, despite nearly complete inhibition of catalase. This would indicate that processes other than catalase activity must contribute to acetaldehyde accumulation.