1-(3-Chlorophenyl)biguanide (m-CPBG) is an agonist of the serotonin (5-HT) receptor subtype 5-HT3.It selectively binds 5-HT3 over 5-HT1A and 5-HT2 receptors (Kis = 0.002, 10, and 10 µM, respectively) but also binds to high and low affinity sites on the dopamine transporter (DAT; IC50s = 0.4 and 34.8 µM, respectively, in rat caudate putamen synaptosomal membranes). m-CPBG induces depolarization of isolated rat vagus nerve and stimulates inositol phosphate formation in rat frontocingulate cortical slices (EC50s = 0.05 and 4.2 µM, respectively). It induces bradycardia, an effect that can be reversed by the 5-HT3 receptor antagonist ondansetron, in anaesthetized cats (ED50 = 20.3 nmol/kg).