Glucose is known to stimulate proinsulin synthesis in pancreatic islets of rats as well as of many other animals, but the mechanism involved remains uncertain. If proinsulin induction by glucose is regulated at the transcriptional level, the amount of proinsulin mRNA may be increased by the administration of glucose. Alternatively, if proinsulin induction is regulated at the translational level, the rate of proinsulin synthesis may not be correlated with the amount of proinsulin mRNA. We have now used a proinsulin cDNA hybridisation method to determine the amount of proinsulin mRNA and the subcellular distribuation of proinsulin mRNA in rat pancreatic islets. The results suggest that glucose-induced proinsulin synthesis is mainly achieved by enhancing the trnaslation efficiency of proinsulin mRNA preexistent on the membrane-bound polysome in pancreatic islets.