This experiment was carried out to investigate the effect of N. sativa L. on histopathology of pancreatic beta-cells, and blood insulin and glucose concentrations in streptozotocin-induced diabetic rats. Fifty male Wistar rats (200-250 g) were divided into two experimental groups (diabetics with no treatment and diabetics with N. sativa L. treatment), each containing twenty-five rats. Diabetes was induced in both groups by a single intraperitoneal injection of streptozotocin (STZ) (50 mg/kg). The experimental animals in both groups became diabetic within 24 hours after the administration of STZ. The rats in N. sativa L.-treated group were given the daily intraperitoneal injection of 0.20 ml/ kg of A sativa L. volatile oil for 30 days starting the day after STZ injection. Control rats received only the same amount of normal saline solution. The rats in both groups received the last injection 24 hours before the sacrification and 5 randomly-selected rats in each group were sacrificed before, and the 1, 10, 20 and 30 days after the STZ injection to collect blood and pancreatic tissue samples. The N. sativa L. treatment caused a decrease in the elevated serum glucose, an increase in the lowered serum insulin concentrations and partial regeneration/ proliferation of pancreatic beta-cells in STZ-induced diabetic rats with the elapse of the experiment. It is concluded that the hypoglycaemic action of N. sativa L. could be partly due to amelioration in the beta-cells of pancreatic islets causing an increase in insulin secretion. More studies are needed to demonstrate the exact mechanism of action of N. sativa L. on ameliorated blood glucose concentration in STZ-induced diabetes. -rats; N. sativa L.; glucose; insulin, beta-cells. (C) 2003 Tohoku University Medical Press.