The DNA synthesis of isolated pancreatic islets has been determined after culture in vitro of the islets for 5 days in media containing either various glucose concentrations or tolbutarnide or cortisone. Only glucose stimulated the replication of DNA, while cortisone was inhibitory and tolbutamide without effect. It is concluded that glucose by itself stimulates the mitotic rate of the islet cells and that the corresponding effect of steroids in vivo may be mediated through the ensuing hyperglycemia. (Endocrinology96: 1051, 1975)