Chemiluminescence induced in isolated islets from rat pancreas by the diabetogenic drugs, alloxan and streptozotocin, has been measured. The assay system consisted of 3 microM of luminol, 10 islets, and 100 microM of alloxan or 500 microM of streptozotocin in 5 ml Krebs-Ringer bicarbonate buffer containing 16 mM of Hepes (pH 7.4). Alloxan-induced chemiluminescence appeared very rapidly and lasted more than 5 min. On the other hand, streptozotocin failed to produce chemiluminescence over a period of 60 min after addition. The presence of superoxide dismutase (1000 U/ml) and/or catalase (100 U/ml) markedly suppressed alloxan-induced chemiluminescence. These results suggest that alloxan acts as an exogenous free radical generator in pancreatic islets, but that streptozotocin does not. The involvement of superoxide anion and hydrogen peroxide in production of chemiluminescence by alloxan suggests that the hydroxyl radical may mediate this chemiluminescence.