Brain sigma-type receptors are thought to mediate hallucinogenic effects of certain benzomorphan opiates in humans. The biochemical characterization of sigma receptors has been difficult because of the lack of potent and selective ligands. We report here the synthesis and characterization of a tritiated, symmetrically substituted guanidine derivative, 1,3-di(2-[5-3H]tolyl)guanidine ([3H]Tol2Gdn), that binds with high affinity to a single population of binding sites in guinea pig brain membrane preparations. The [3H]Tol2Gdn binding site displays stereoselectivity for dextrorotatory optical isomers of benzomorphan opiates known to have sigma-type behavioral effects. Furthermore, the [3H]Tol2Gdn binding site has a high affinity for haloperidol and for phenothiazine antipsychotics, which have antihallucinatory properties in humans. The drug-selectivity profile of [3H]Tol2Gdn binding closely correlates with the drug-selectivity profile of tritiated (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [+)-[3H]3-PPP) binding to guinea pig brain membrane receptors. (+)-[3H]3-PPP has been proposed to be a selective sigma-receptor ligand [Largent, B. L., Gundlach, A. L. & Snyder, S. H. (1984) Proc. Natl. Acad. Sci. USA 82, 4983-4987]. Receptor autoradiography using [3H]Tol2Gdn on slide-mounted rat and guinea pig brain sections reveals a heterogeneous distribution pattern of enriched binding in limbic and sensorimotor structures of the brain. These results indicate that [3H]Tol2Gdn is a selective ligand for the sigma-site. Availability of this sigma-receptor probe should greatly facilitate the physiological, biochemical, and pharmacological characterization of sigma receptors in brain.