Previous studies of plasma taurine concentrations in epileptics have yielded equivocal results. We measured plasma and urinary taurine in 41 epileptic and 68 control subjects and found plasma concentrations among epileptics to be comparable in general to those of controls, but that two or three classes of plasma taurine concentrations, possibly genetically regulated, occur in both epileptic and control subjects. Our previous studies and data from the present study on taurine excretion revealed three excretion classes under genetic control. The principal finding is that epileptics include disproportionate numbers of low excretors (high reabsorbers), who are presumptive homozygotes for the allele effecting higher reabsorption. If confirmed, these findings suggest that the transport of taurine, rather than absolute taurine concentration, may explain the efficacy of taurine administration in some epileptics but not in others. The locus involved may be one component in the polygenic diathesis to the idiopathic epilepsies.