Abstract: AIM: To study the effect of berberine hydrochloride(Ber) on muscarinic receptors. METHODS: Use isolated arotic rings, isolated helical trachea strips and radioligand-receptor binding experiment. RESULTS: The inhibitory action of acetylcholine(ACh) on the contractile response of KCl in the endothelium-intact thoracic arotic rings of rats was potentiated by Ber concentration-dependently. At concentrations above 20 μmol.L^-1, Ber relaxed the endothelium-intact rings directly. The EC₅₀ value was 31.5 μmol.L^-1. But such inhibitory action of Ber was totally abolished by removing the endothelium or by pretreatment with atropine 10 μmol.L^-1. Ber dose-dependently induced contraction on isolated helical strips of guinea pig trachea. This action was completely blocked by atropine 10 μmol.L^-1. In the radioligand-receptor binding experiments, Ber was shown to bind to muscarinic receptors with definite affinity in rat brain. The Ki value was 1.6 μmol.L^-1. CONCLUSION: Ber is an agonist of muscarinic receptor. It may release EDRF from vascular endothelia cells by exciting muscarinic receptors to relax arotic rings.