Abstract Withania coagulans fruit has been shown to possess antihyperglycemic properties and is used in the traditional Indian system of medicine. However, there has no systematic study of its mechanism of action. In a rat model diabetes mellitus (DM) was induced by intraperitoneal injection of nicotinamide (230?mg/kg of body weight) followed by streptozotocin at 55?mg/kg of body weight. After 96?h, mildly diabetic (MD) (fasting plasma glucose [FPG]=7-11.1?mmol/L) and severely diabetic (SD) (FPG>11.1?mmol/L) rats were treated with aqueous extract of W. coagulans fruit at doses of 125, 250, and 500?mg/kg of body weight/day orally. FPG, postprandial plasma glucose (PPPG), glycosylated hemoglobin (HbA(1c border=0>, plasma insulin, tissue glycogen, and glucose-metabolizing enzymes were assayed at Day 30. Treatment of diabetic animals (MD and SD) with different doses of aqueous W. coagulans resulted in significantly decreased FPG, PPPG, and HbA(1c) (P<.01), whereas serum insulin increased significantly compared with that in diabetic-untreated rats (P<.01). MD and SD animals treated with aqueous W. coagulans also showed significant increases in liver and muscle glycogen compared with diabetic-untreated animals (P<.01). Moreover, activities of glucokinase and phosphofructokinase were also significantly increased (P<.01), whereas glucose-6-phosphatase activity was significantly decreased (P<.01) in MD and SD groups treated with aqueous W. coagulans compared with diabetic-untreated groups. The most effective dose of aqueous W. coagulans was 250?mg/kg of body weight. These results show that the aqueous extract of W. coagulans fruit has significant antihyperglycemic effects, which may be through the modulation of insulin levels and related enzyme activities.