We previously reported that new β cells differentiated in pancreatic islets of mice in which diabetes was produced by injection of a high dose of the β cell toxin streptozotocin (SZ), which produces hyperglycemia due to rapid and massive β cell death. After SZ-mediated elimination of existing β cells, a population of insulin containing cells reappeared in islets. However, the number of new β cells was small, and the animals remained severely hyperglycemic. In the present study, we tested whether restoration of normoglycemia by exogenous administered insulin would enhance β cell differentiation and maturation. We found that β cell regeneration improved in SZ-treated mice animals that rapidly attained normoglycemia following insulin administration because the number of β cells per islet reached near 40% of control values during the first week after restoration of normoglycemia. Two presumptive precursor cell types appeared in regenerating islets. One expressed the glucose transporter-2 (Glut-2), and the other cell type coexpressed insulin and somatostatin. These cells probably generated the monospecific cells containing insulin that repopulated the islets. We conclude that β cell neogenesis occurred in adult islets and that the outcome of this process was regulated by the insulin-mediated normalization of circulating blood glucose levels.