The activities of the hepatic glycolytic enzymes glucokinase (GKase) and hexokinase (HKase) in herbivorous Microtus arvalis were very low and the hepatic fructose-1,6-diphosphatase (FDPase) activities were almost the same as those in C57BL/6J mice. Glycosuria was observed in over 50% of voles fed on a low fibre, high concentrate diet. Voles with a high incidence of glycosuria for over 6 weeks became insulin deficient. In these diabetic voles, the hepatic GKase, HKase and FDPase activities decreased considerably as a result of diminished insulin secretion and fatty degeneration of the hepatic cells. It was considered that M. arvalis would be a useful animal model in which to study disorders of glucose utilization in herbivora.