The effect of bombesin against injury on rat islet B cells was studied in three kinds of experiments: (1). In vivo experiment, it was found that preinjection of bombesin (50 micrograms/kg, sublingual v.) could effectively prevent an increase of plasma glucose and decrease of plasma insulin in diabetic rat induced by alloxan (200 mg/kg, s.c.) (2). In vitro experiment, isolated pancreas perfusion showed that alloxan-induced (14 mmol/L) perfusion fluid inhibition of insulin secretion could be reversed by pretreatment of bombesin (10(-3) mmol/L). (3). Investigation on isolated and incubated islets demonstrated that alloxan induced decrease of insulin and somatostatin secretion and increase of glucagon secretion could be prevented by bombesin. The above-mentioned results suggest that bombesin may play an important role in the regulation of plasma glucose in diabetic rat and have a potent preventive effect against the development of diabetes.