This study, carried out in human cerebral cortical membranes, confirms previous findings in human substantia nigra that serotonin-5-O-carboxymethyl-glycyl[I-125]tyrosinamide ([I-125]GTI) labels a homogeneous population of recognition sites consistent with a 5-HT1D receptor pharmacology. In addition the results indicate that, under the assay conditions described, [I-125]GTI specifically labels the 5-HT1Dbeta recognition site since ketanserin and ritanserin display a low affinity consistent with their activities at this subtype of the 5-HT1D receptor.