Selection for and against diabetes and subsequent inbreeding of Chinese hamsters started in 1963. Currently there are six inbred sublines that have greater than 85% incidence of glycosuria and two control inbred nondiabetic sublines that are essentially free of glycosuria. At birth, hamsters from inbred sublines are considered prediabetic. There is phenotypic variation between diabetic sublines. Onset time, incidence of ketonuria, blood glucose, plasma insulin, glucagon and glycohydrolase levels vary from subline to subline, but pancreatic insulin and glucagon levels are consistently low and high, respectively, in all diabetic sublines compared with nondiabetics. Experimental breeding data suggest a minimum of two homozygous recessive genes for diabetes. It is not known if the inbred lines are similar diabetic genotypes, but the probability is high that modifier background genes vary from subline to subline. Chinese hamsters have diabetes ranging from mild to severe. Animals weighing 25 g can excrete up to 75 ml of urine containing 3 g of glucose per day. Fasting blood glucose as high as 500 mg/dl and 10 mumol/ml of beta-hydroxybutyrate have been reported. Gluconeogenesis is elevated, and some glycolytic enzymes are decreased in severe diabetes. Low levels of renal acid glycohydrolase enzymes may contribute to glomerular capillary loop basement membrane thickening in diabetic hamsters. Caloric restriction per se or reduction of dietary fat prevented onset of hyperglycemia and hyperinsulinemia in prediabetics. Morphologic changes have been observed in pancreatic islets, kidney, nerve, blood vessels, eyes, brain, and genito-urinary systems of diabetic Chinese hamsters. Pathogenesis of diabetes in this animal appears to be related to an increased demand for insulin. Initially there is a positive response to this demand by beta cells, but exhaustion occurs. This is followed by a decrease in beta-cell mass and relative or absolute insulin deficiency.