Administration of monosodium aspartate (MSA) to neonatal voles, Microtus arvalis Pallas induced neuronal necrosis in several brain regions including the arcuate and ventromedial hypothalamic nuclei. In the growing stage, 56.7% showed glycosuria. In the mild diabetic voles, blood glucose concentrations rose to 180 mg/dl (72 mg/dl in normal), and plasma insulin levels were also increased to 112 microU/ml (21 microU/ml in normal). In the severe diabetic voles, on the other hand, blood glucose concentrations rose over 250 mg/dl, and plasma insulin levels decreased under 10 microU/ml. Histopathological examination of the pancreatic islets revealed moderately enlarged islets, and nuclear hypertrophy and partial degranulation of B cells in the mild diabetic voles, and moderately to markedly enlarged islets, and marked degranulation, vacuolation and accumulation of glycogen granules of B cells in the severe diabetic voles. These results indicated that the diabetic syndrome in herbivorous voles was induced by administration of MSA and diabetic voles fell into marked insulin deficiency in the severe stage.