Oral anti-diabetic agents with hypoglycemic action via mechanisms distinct from the sulfonylureas have recently been developed. One of these, alpha-glucosidase inhibitor slows the absorption rate of carbohydrate from the small intestine. Effects of voglibose on glycemic control and on the function of pancreatic islets were evaluated using Goto-Kakizaki (GK) rats with non-insulin-dependent diabetes mellitus (NIDDM). Insulin secretory capacity in response to glucose of islets was significantly improved after 8-week administration of voglibose. The treatment increased the insulin content of the islets to almost twice that in untreated controls. Thus, the treatment can restore the deteriorated islet function of GK rats, possibly through protection from glucose toxicity.