Hereditary insulin-deficient diabetes mellitus occurs in certain sublines of nonobese Chinese hamsters. Several characteristics of this syndrome are similar to those seen in insulin-deficient human diabetics. Therefore, to characterize pancreatic islet function, dynamic insulin and glucagon release from normal and nonketotic diabetic hamster pancreases in response to glucose (300 mg/100 ml) and theophylline (10 mM), infused singly and together, was studied in vitro.20-min glucose infusions of normal hamster pancreases caused biphasic insulin release, consisting of a rapid first peak and a gradually rising second phase, similar to that reported for man in vivo. Both phases were significantly reduced in the diabetic pancreases. Theophylline alone stimulated similar nonphasic insulin release in both the normal and the diabetic pancreases. Glucose and theophylline together caused greater insulin release than either stimulant alone in both normals and diabetics; however, the diabetic response was still subnormal. Glucose suppressed glucagon release from normal pancreases; suppression was significantly impaired in diabetics. Theophylline stimulated nonphasic glucagon release in both the normals and diabetics. Glucose partially suppressed the theophylline-stimulated release in both groups.Insulin/glucagon molar ratios of the diabetics were consistently subnormal, although individual hormone levels often overlapped into the normal range. IN SUMMARY, THE PANCREASES OF GENETICALLY DIABETIC CHINESE HAMSTERS PERFUSED IN VITRO SHOWED: (a) decreased first and second phase insulin release in response to glucose-containing stimuli-only partially ameliorated by theophylline-, and (b) impaired suppression of glucagon in response to glucose, resulting in (c) a decreased insulin/glucagon molar ratio. These data support the suggestion that both alpha and beta cells of diabetic pancreases may be insensitive to glucose.