1. The pharmacological profile of niaprazine was investigated using in vitro ligand binding techniques. 2. Niaprazine exhibits a low affinity for the vesicular monoamine transporter and for D2, alpha 2, beta, H1 and muscarinic cholinergic receptors. Niaprazine, particularly the (+)stereoisomer, has a higher affinity for alpha 1 (Ki = 77 nM) and 5-HT2 (Ki = 25 nM) binding sites, but is poorly recognized by 5-HT1A and 5-HT1B binding sites (Ki sigma mciroM). In contrast, p-fluoro-phenylpiperazine, a major metabolite of niaprazine, exhibits a higher affinity for the 5-HT1 subclasses than for the 5HT2 class. 3. These results suggest that the pharmacological properties of niaprazine reflect both its non-reserpinic catecholamine depletor effect and its action on alpha 1 and 5-HT2 receptors. A role of p-fluoro-phenylpiperazine via 5-HT1 sites cannot be excluded.