The aim of this study was to evaluate if the impaired insulin production of the beta-cell deficient islet organ of neonatally streptozotocin (SZ) injected rats is caused by exposure of the beta-cells to a long-lasting functional demand in vivo or a persistent toxic effect of the drug. For this purpose islets were isolated from adult rats which had received an ip injection of SZ (100 mg/kg body weight) on postnatal day 1 or from control rats receiving the solvent only. The islets used were either fresh or after culture for 2, 7, or 14 days in RPMI 1640 supplemented with 5.6, 11.1, or 16.7 mM glucose. After the various culture periods determinations were performed of the islet contents of insulin and insulin mRNA and the rates of (pro)insulin biosynthesis and insulin release. Freshly isolated islets from SZ-treated rats exhibited lower contents of insulin and insulin mRNA, a lower rate of (pro)insulin biosynthesis, and an impaired glucose-sensitive insulin release. Similar results were obtained after 2 days of culture, in each of the glucose concentrations. After 7 days of culture, however, the content of insulin mRNA and the rate of (pro)insulin biosynthesis of the SZ islets were restored to the control levels. When such islets were cultured for 7 days in 5.6 mM glucose, they exhibited a glucose-sensitive insulin release similar to that of the control islets. A difference in the insulin release between the two groups nevertheless persisted after culture for 7 days at either 11.1 and 16.7 mM glucose. Also, after 14 days of culture at 16.7 mM glucose there was an impaired glucose-sensitive insulin release from SZ islets, while islets cultured at 11.1 mM glucose showed a glucose-stimulated insulin release similar to that of the controls. The present data indicate that, as far as storage and biosynthesis of insulin is concerned, the functional aberrations observed in the freshly isolated SZ-islets did not reflect a permanent cytotoxic damage. The persistent impairment of insulin release after culture at 16.7 mM glucose may reflect either an injurious effect of the mildly diabetic metabolism in vivo or of the neonatal streptozotocin injection.