Several kinds of medicinal plants have been used to treat diabetes mellitus. In the present study, Thunbergia laurifolia Linn. (purple flower strain) were tested for hypoglycemic activity in rats. The effects on the reproductive system and the histology of the pancreas were also investigated. The hypoglycemic properties of the aqueous extract from the leaves (60 mg/ml/day) were evaluated in normoglycemic and alloxan-induced diabetic rats. The results showed that high blood glucose levels of diabetic rats were associated with severe destruction of beta-cells (insulin-secreting cells) in the islet of Langerhans. The 15-day-treatment of T. laurifolia extract decreased levels of blood glucose in diabetic rats. The recovery of some beta-cells was found in diabetic rats treated with the extract, although not completely normal. Whether T. laurifolia leaf contains insulin-like substance(s) which directly act as hypoglycemic agents, or contains substances that induce the regenerative process of beta-cells remains to be further investigated. Alteration of the reproductive system was also observed in diabetic rats, but it was not improved by treatment with T. laurifolia extract.