Abstract Iloperidone has demonstrated an interesting monoamine receptor profile in radioligand binding studies, with nanomolar affinity for certain noradrenaline, dopamine, and serotonin receptors. In this study, the agonist/antagonist activity of iloperidone was determined in cell lines expressing recombinant human D 2A, D 3, α 2C, 5-HT 1A, or 5-HT 6 receptors. With the exception of 5-HT 6 receptors, these receptors are negatively coupled to cyclase. Thus, after stimulation with forskolin, the agonists dopamine (at D 2A and D 3), noradrenaline (at α 2C), or 8-OH-DPAT (at 5-HT 1A) induced a reduction in cAMP accumulation. Conversely, activation of the 5-HT 6 receptor by 5-HT led to an increase in cAMP accumulation. Iloperidone alone was devoid of significant agonist activity but inhibited the agonist response in all 5 cell lines in a surmountable and concentration-dependent fashion. Iloperidone was most potent at D 3 receptors (pK B 8.59 ± 0.20; n = 6), followed by α 2C (pK B 7.83 ± 0.06; n = 15), 5-HT 1A (pK B 7.69 ± 0.18; n = 10), D 2A (pK B 7.53 ± 0.04; n = 11) and 5-HT 6 (pK B 7.11 ± 0.08; n = 11) receptors.