Factors affecting the inhibition of ox brain glutamate dehydrogenase (GDH) by the antipsychotic drug perphenazine have been studied. Inhibition was found to be of mixed type with respect to 2-oxoglutarate and competitive towards NADH. However, the data indicate that perphenazine binds to a site distinct from the catalytic site to which NADH binds. Perphenazine also enhanced the high-substrate inhibition by these two substrates. Inhibition by perphenazine was not affected by the allosteric effector GTP but it was enhanced by increasing pH, in the range of 6.3 to 7.6, and diminished by increasing ionic strength. Low concentrations of perphenazine relieved the inhibition of GDH by phosphatidylserine and cardiolipin. However, at higher concentrations phosphatidylserine did not interfere with the inhibition by perphenazine whereas cardiolipin relieved it. The possible significance of these interactions in terms of the behaviour of this antipsychotic drug in vivo are discussed.