We investigated the impact of olmesartan and temocapril on pancreatic islet beta-cells during the development of diabetes mellitus using Otsuka-Long-Evans-Tokushima Fatty (OLETF) rats. Four-week-old male OLETF rats were fed standard chow (untreated: n = 5), or chow containing either 0.005% olmesartan (n = 5) or 0.01% temocapril (n = 5) until being sacrificed at 35 weeks of age. Pancreas sections were double-stained with anti-insulin and anti-glucagon antibodies. The percent areas of beta-cells, alpha-cells and non-alpha-non-beta-cells were compared among groups. In untreated OLETF rats, the fasting plasma glucose (FPG) level was elevated at the 18th week and remained elevated until the 35th week. On the other hand, no significant elevation in FPG levels was observed in otmesartan- or temocapril-treated rats. Pancreatic islets from olmesartan-treated rats were significantly smaller in size as compared with those from untreated OLETF rats. Furthermore, the average area occupied by beta-cells as a fraction of the total area of an individual islet was significantly higher in olmesartan- or temocapril-treated rats than that in untreated OLETF rats. Olmesartan and temocapril both prevented the development of hyperglycemia, possibly through the prevention of islet beta-cell loss in spontaneously diabetic OLETF rats.