Succinic semialdehyde dehydrogenase and NADPH-dependent aldehyde reductase have been purified from ox brain by affinity chromatography on 5′ AMP–Sepharose and 2′5′ ADP–Sepharose respectively. Aldehyde reductase has also been purified using chromatography on Procion Red HE3B–Sepharose. The effects of the anticonvulsant drug sodium valproate were examined on these enzymes, and also on GABA-aminotransferase partially purified from ox brain. Aldehyde reductase was inhibited by valproate in an uncompetitive manner with respect to aldehyde substrates (Ki= 38–85 μM). The inhibitions of succinic semialdehyde dehydrogenase and GABA aminotransferase were approx 2 orders of magnitude weaker, suggesting that aldehyde reductase may be an important site of action of anticonvulsant drugs.