The water extract of Ganoderma lucidum was found, by the oral glucose tolerance test (OGTT), to reduce blood glucose and insulin 10 min after oral glucose infusion as compared to control rats. In addition, the water extract increased blood insulin 30 min and 60 min after oral glucose infusion without affecting blood glucose level as compared to control rats. Intravenous infusion of adrenaline with the oral glucose tolerance test (OGTT) caused an increase of blood glucose without elevating blood insulin level, as found by the oral glucose tolerance test (OGTT). The water extract of G. lucidum reduced the elevation of blood glucose without elevating blood insulin by the intravenous infusion of adrenaline and oral infusion of glucose.