The anticonvulsant valproic acid (VPA, 2-n-propylpentanoic acid) causes inhibition of the citric acid cycle and elevations of central nervous system (CNS) gamma-aminobutyric acid (GABA) levels, which correlates with anticonvulsant action. No unifying mechanism for these actions of VPA has won general acceptance. alpha-Ketoglutarate dehydrogenase complex (KDHC) is a critical control enzyme in the CNS. We hypothesized that VPA may be an inhibitor of this enzyme since decreased KDHC activity would reduce substrate flux through the citric acid cycle and may increase flux into GABA synthesis. To test this hypothesis, inhibition of purified beef brain KDHC by VPA and its metabolites 2-n-propylpent-2-enoic acid (delta 2,3 VPE) and their coenzyme A (CoA) derivatives were studied. Preincubation of the NADH-reduced enzyme with delta 2,3 VPE, VPA-CoA, and delta 2,3 VPE-CoA caused time-dependent inactivation, reversible by addition of CoA. Under steady-state conditions, delta 2,3 VPE and VPA-CoA were competitive inhibitors of KDHC and delta 2,3 VPE-CoA was a mixed inhibitor. These observations have implications for the molecular mechanisms of VPA action. VPA derivatives cause inactivation and inhibition of KDHC, which may explain the anticonvulsant and some toxic actions of VPA.