We investigated the plasma glucose-lowering mechanism(s) of Rh2, a ginsenoside derived from Panax ginseng, in rats with streptozotocin-induced diabetes (STZ-diabetic rats). After intravenous injection over 120 min into fasting STZ-diabetic rats, Rh2 decreased plasma glucose in a dose-dependent manner. In parallel to the lowering of plasma glucose, an increase of plasma beta-endorphin-like immunoreactivity was observed. In addition, naloxone and naloxonazine at doses sufficient to block opioid mu-receptors inhibited the plasma glucose-lowering action of Rh2 in genetically wild-type, diabetic mice. In contrast, Rh2 failed to lower plasma glucose in opioid mu-receptor knockout diabetic mice. An increase in gene expression at both the mRNA and protein levels of glucose transporter subtype 4 (GLUT 4) was observed in soleus muscle obtained from STZ-diabetic rats treated with Rh2 three times daily for one day; this increase in expression was absent when opioid mu-receptors were blocked. In conclusion, our results suggest that ginsenoside Rh2 may lower plasma glucose in STZ-diabetic rats based on an increase in beta-endorphin secretion that activates opioid mu-receptors thereby resulting in an increased expression of GLUT 4.