Phenytoin was observed to inhibit competitively the sodium dependent high affinity synaptosomal transport of both glutamate (Glu) and gamma-aminobutyric acid (GABA) with Ki values of 66 +/- 10 and 185 +/- 65 microM, respectively. This contrasted with a previous report that the uptakes of Glu and GABA were enhanced by phenytoin. The degree of inhibition is dependent on the concentrations of the competing drug and substrate present. Taking the therapeutic levels of phenytoin and the overall brain Glu and GABA contents, the degrees of inhibition obtainable appear to be negligible. However, as most of the high levels of Glu and GABA in the brain are intracellular, Glu, and GABA concentrations in the microenvironment of the uptake sites may be sufficiently small so that the ability of phenytoin to inhibit Glu and GABA transport may contribute significantly to the anticonvulsant property of this drug.