To further clarify the role of 5-hydroxytryptamine (5HT) in the behavioral effects of tetrahydro-beta-carboline,male rats were trained to discriminate either 20 mg/kg THBC from its vehicle (n= 10) or 2.0 mg/kg fenfiuramine from saline (n=5). THBC was observed to produce fenfluramine-like effects in the fenfluramine trained rats while fenfluramine produced THBC-Iike effects in the THBC-discriminating rats. To investigate which of the serotonergic receptors may mediate the THBC-induced discriminative stimulus, various putatively specific 5HT agonists were administered to THBC-trained rats. Results indicate that the 5HT1Breceptor agonists TFMPP and m-CPP substitute for THBC in a dose-response manner whereas 5HT~A agonists do not generalize to the THBC-induced discriminative stimulus. These observations support a role for the 5HT~Rreceptor site in the discriminativestimulus properties of THBC.