1. The aim of the present study was to clarify the role of ginsenoside Rh2 as the active compound in Panax ginseng root for lowering plasma glucose in animals. 2. Plasma glucose was assessed using the glucose oxidase method. Changes in the levels of insulin and C-peptide in plasma were measured by ELISA using commercially available kits. 3. After intravenous injection into fasting Wistar rats for 60 min, ginsenoside Rh2 (0.1-1.0 mg/kg) decreased plasma glucose in a dose-dependent manner. In parallel with the decrease in plasma glucose, increases in plasma insulin levels, as well as plasma C-peptide, were observed in rats receiving the same treatment. These effects of Rh2 were reversed by atropine (0.1-1.0 mg/kg), but not affected by the ganglionic nicotinic antagonists pentolinium or hexamethonium (both at 7.5 mg/kg). 4. Disruption of synaptically available acetylcholine (ACh) using an inhibitor of choline uptake (hemicholinium-3; 1-10 microg/kg) or an inhibitor of vesicular ACh transport (vesamicol; 1.5-3.5 mg/kg) abolished the actions of Rh2. In addition, physostigmine (0.1-0.5 mg/kg), at a concentration sufficient to inhibit acetylcholinesterase, enhanced the actions of the ginsenoside Rh2. Thus, mediation of the effects of Rh2 to enhance insulin secretion by ACh released from nerve terminals can be considered. 5. Blockade of the increase in plasma insulin and the plasma glucose-lowering action of Rh2 by 4-diphenylacetoxy-N-methylpiperdine methiodide (4-DAMP; 5-10 microg/kg) indicates the participation of muscarinic M(3) receptors. Increases in plasma C-peptide level induced by Rh2 were also sensitive to 4-DAMP. 6. The results of the present study suggest that ginsenoside Rh2 has the ability to increase insulin secretion as a result of the release of ACh from nerve terminals that then stimulates muscarinic M(3) receptors in pancreatic cells. This finding shows the mechanism for the plasma glucose-lowering action of ginsenoside Rh2, that is one of the major principles contained in P. ginseng root. Thus, ginsenoside Rh2 may be applied as an adjuvant for the management of diabetes.