Phenylbiguanide is a 5-HT₃ receptor selective agonist with an EC₅₀ of 3.0±0.1 μM. IC50 & Target: EC50: 3.0±0.1 μM (5-HT₃ Receptor)^[1] In Vitro: Phenylbiguanide (1-Phenylbiguanide) is a 5-HT₃ receptor selective agonist with pEC₅₀s of 5.57, 4.07, and 4.47 for r5-HT_3A(b), h5-HT_3A, m5-HT_3A(b) , respectively^[1]. The effect of Phenylbiguanide (1-Phenylbiguanide hydrochloride) in promoting the growth of the HT29 cell line is investigated. Phenylbiguanide causes a dose dependent proliferation of HT29 cells after 48 hours incubation. The maximum proliferation is at a 5HT concentration of 12.5 μM (P≤0.01). Phenylbiguanide significantly stimulates the growth of cells at concentrations of 3.125 μM (P≤0.05) and 6.25 μM (P≤0.01)^[2]. In Vivo: In anaesthetised mice, Phenylbiguanide (PBG), a drug that is known to stimulate cardiopulmonary afferent C-fibres,is injected into the right atrium of the heart and mapped c-Fos expression within specific regions of the central nervous system. Intraatrial injection of PBG produces a reflex cardiorespiratory response including a pronounced bradycardia and a respiratory depression^[3].