Arylpiperazines, such as 1-(3-trifluoromethylphenyl)piperazine (TFMPP) and its chloro analogue mCPP, are 5-HT1 agonists, whereas quipazine, i.e., 2-(1-piperazino)quinoline, appears to be a 5-HT2 agonist. Radioligand binding studies using rat cortical membrane homogenates and drug discrimination studies using rats trained to discriminate a 5-HT1 agonist (i.e., TFMPP) or a 5-HT2 agonist (i.e., 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM)) from saline reveal that quipazine and its 1-deaza analogue 2-naphthylpiperazine (2-NP) bind at 5-HT1 and 5-HT2 sites but produce stimulus effects similar to those of DOM. A structurally related compound, 1-naphthylpiperazine (1-NP), possesses a high affinity for 5-HT1 (Ki = 5 nM) and 5-HT2 (Ki = 18 nM) sites. 1-NP produces stimulus effects similar to those of TFMPP and is able to antagonize the stimulus effects produced by DOM. The present results suggest that the unsubstituted benzene ring of quipazine, and of its 1-deaza analogue 2-naphthylpiperazine, makes a significant contribution to the binding of these agents to 5-HT2 sites and, more importantly, may account for their 5-HT2 agonist properties.