Three human post-mortem brains were dissected into seventeen areas and assayed for aldehyde dehydrogenase (EC 1.2.1.3) activity employing two assay systems: one at 68 microM and another at 13.6 mM propionaldehyde. The levels of activity with 68 microM propionaldehyde were significantly higher in cerebellum and putamen. The same brain areas were also examined by isoelectric focusing. By this procedure two distinct bands of aldehyde dehydrogenase activity (the cytoplasmic E1 and mitochondrial E2) could be readily visualized in cerebellum and putamen while other brain areas contained mainly the mitochondrial E2 isozyme.