OBJECTIVE To investigate the hypoglycemic effect of Ganoderma lucidum polysaccharides (Gl-PS) in the normal fasted mice and its possible mechanism. METHODS Normal fasted mice were given a single dose of Gl-PS 25, 50, and 100 mg/kg by i.p. and the serum glucose was measured at 0, 3, and 6 h after administration. Gl-PS 100 mg/kg were also given by i.p. and the serum glucose and insulin levels were measured at 0 min, 30 min, 1 h, 3 h, 6 h, and 12 h. Pancreatic islets were isolated and incubated with glucose 5.6 mmol/L and different concentration of Gl-PS, the insulin content of islets and insulin release were examined. The islets fluorescent intensity of [Ca2+]i was also studied with a confocal microscope. Verapamil and egtazic acid were used to testify whether the insulin-releasing effect of Gl-PS was mediated by its ability to raise the Ca2+ influx. RESULTS Gl-PS dose-dependently lowered the serum glucose levels at 3 h and 6 h after administration. Gl-PS 100 mg/kg raised the circulating insulin levels at 1 h after administration. In vitro, Gl-PS had no effect on islets insulin content, but it stimulated the insulin release after incubation with glucose 5.6 mmol/L. Confocal microscope showed that Gl-PS 100 mg/L had the capacity to raise the [Ca2+]i. The insulin-releasing effect of Gl-PS was inhibited by verapamil/egtazic acid. CONCLUSIONS Gl-PS possesses the hypoglycemic effect on normal mice; one mechanism is through its insulin-releasing activity due to a facilitation of Ca2+ inflow to the pancreatic beta cells.