Plasma and urinary taurine were measured in 41 epileptic and 68 control subjects. Plasma concentrations among epileptics were comparable to those of controls, but 2 or 3 classes of plasma taurine concentrations, possibly genetically regulated, occur in epileptic and control subjects. Data on taurine excretion revealed 3 excretion classes under genetic control. Epileptics include disproportionate numbers of low excretors (high reabsorbers), who are presumptive homozygotes for the allele effecting higher reabsorption. Taurine transport, rather than absolute taurine concentration, may explain the efficacy of taurine administration in some epileptics but not in others. The locus involved may be 1 component in the polygenic diathesis to the idiopathic epilepsies.