The affinity of 1-(m-chlorophenyl)piperazine (mCPP) for 11 neurotransmitter receptor binding sites was determined in human brain membranes. mCPP is essentially equipotent at all 5-hydroxytryptamine (5-HT) receptor subtypes (IC50 values ranging from 360 to 1300 nM). The drug displays similar affinity (IC50 = 570 nM for alpha 2-adrenergic receptors labeled for 3H-rauwolscine. mCPP is less potent at alpha 1- and beta-adrenergic, dopamine, and muscarinic cholinergic receptors (IC50 values 2500-24,000 nM). mCPP is inactive at both benzodiazepine receptors and the 5-HT uptake sites at concentrations below 100,000 nM. These data demonstrate that mCPP displays similar potency for multiple neurotransmitter receptor binding sites in human brain.