This study was performed to observe the effects of acarbose and Rumex patientia on morphological change of pancreatic B cells in streptozotocin (STZ)-induced diabetic (type 2) rats. Two-day-old Wistar albino rats were intraperitoneally injected with 100 mg/kg of STZ or vehicle alone for control. Vehicle and STZ given rats were divided into six groups (1st, 2nd and the 3rd groups are control; the 4th, 5th and 6th groups are STZ groups). The 1st and the 4th groups received water, the 2nd and the 5th groups received 40 mg acarbose/100 g feed, the 3rd and the 6th groups received 2% decoction of Rumex patientia grain. During experimentation period, blood glucose levels were checked periodically, and HbA1c level was measured from cardiac blood at the end of the experiment. Pancreas tissues were examined by electron microscope. Glucose and HbA1c levels increased by STZ were decreased by acarbose and Rumex patientia. Morphologically, we found a mitochondrial vacuolization and swelling as well as dilatation of the endoplasmic reticulum in the B cells of STZ-induced diabetic rats. Also, a decrease in the secretory granules of B cells was observed in the STZ-induced diabetic group. No pathological changes were observed in the STZ + acarbose group. In the STZ + Rumex patientia group, a weak swelling in the B cells was observed in the some of the mitochondria. (c) 2005 Elsevier Ireland Ltd. All rights reserved.