Sixty-seven mongolian gerbils (Meriones unguiculatus), which seem to resemble the sand rats in the manifestation of diabetic syndrome, were used for observations on the development of diabetic process and on the effect of the long term administration of tolbutamide on it. The mean blood glucose (158.2 +/- 14.7 mg/100 ml), mean serum NEFA (0.70 +/- 0.34 mEq/l) and mean serum IRI (68.9 +/- 10.2 muU/ml) in mongolian gerbils were relatively higher than those in rats. Diabetes-like processes found in the pancreas of mongolian gerbils were roughly grouped into two main types: (1) the histopathological process in which the fibrosis, initially prominent at perivascular space, spread to the islets, finally producing the islet's cell degeneration, and (2) the process of diffuse islet's hyperplasia. These two processes were considered to differ from each other, the former being affected by tolbutamide and the latter remaining unaffected. After breeding on the diet containing 0.1 -0.2% tolbutamide for 6 months, there was a general inclination of an increase of cases of severe pathological findings, and the distribution probabilities of the cases carrying the severe fibrotic process in heart, that is, the process from endothelial cell swelling, medial fibrosis and obliterative changes in cardiac arteries which was resulting in the focal degeneration of cardiac muscle, was significantly higher than in the non-treated group. It was concluded that mongolian gerbil might be recommended as a model of experimental diabetes and that tolbutamide seemed to have some harmful effects on the diabetes-like process in mongolian gerbils.