Isolated rat islets were maintained in a simple static incubation system and were exposed to alloxan for a period of five minutes. Alloxan inhibited subsequent glucose-induced insulin release in a dose-dependent manner at 37 degrees C., with 650 muM alloxan producing 94 per cent inhibition of insulin release. Barbituric acid, a compound structurally related to alloxan, provided complete protection (at 37 degrees C.) against this inhibition of insulin release when present during the alloxan exposure. At 23 degrees C., barbituric acid was shown to be absent from the intracellular space of the islet yet still protected completely against alloxan inhibition of insulin release. Thus, barbituric acid apparently provided protection against alloxan in the extracellular medium. By fluorometric and chromatographic analyses, it was determined that barbituric acid reacted rapidly with alloxan to produce a new compound. These findings indicate that barbituric acid protected against alloxan by a chemical reaction in the medium.