This investigation was carried out to evaluate the effect of the extract of Urtica dioica leaves on hyperglycemia and quantitative changes of b-cells in streptozotocin-diabetic rats. Forty male Wistar rats were allocated in groups of normal, diabetic, treatment and protective. Hyperglycemia induced by administrating one dose of 80 mg/kg streptozotocin (STZ) intraperitoneally. Animals in treatment group received Urtica dioica (100 mg/kg/day) for 4 weeks intraperitoneally, one week after injection of STZ. In protective group animals received U. dioica (100 mg/kg/day) for 5 days before inducing diabetes. After five weeks the animals were sacrificed and whole pancreas removed. Pancreas specimens were used for quantitative morphometric analysis after Chromealum hematoxiline - phloxine staining. The mean ± SE of b-cells in non hyperglycemic animals in protective group was higher than in hyperglycemic animals in the same group (54.33 ± 2.4 versus 1.25 ± 0.5, P<0.05). Hyperglycemia was improved in 6 (60%) of rats in protective group and 1 (10%) rat in treatment group OR=0.07 (CI 95%: 0.0-1.1, p=0.06). The logistic regression analysis showed an association between decrease of blood glucose, increase of number of b-cells and administration of Urtica before induction of diabetes. This study showed proliferation of b-cells when of the U. dioica leaves extract (100 mg/kg/day) administrated before induction of diabetes in animal model.