A possible role for insulin in stimulating islet beta-cell replication was examined in neonatal rat pancreatic monolayer cultures. Addition of insulin to serum-free medium increased the mitotic index and stimulated dose-dependent increases in [3H]-thymidine incorporation in nuclei of islet beta-cells in aldehyde-thionine-stained autoradiographs. The effects of insulin were not associated with any significant changes in glucagon or somatostatin levels in the culture media. Multiplication stimulating activity (MSA), an insulin-like growth factor, was about 100-fold more potent than insulin: 3 ng/ml MSA stimulated a half-maximal increase in thymidine labeling of beta-cell (+63%, P less than 0.005), whereas 300 ng/ml insulin was required for a similar effect. The maximal effects of insulin and MSA were similar, and the combination of maximal stimulatory concentrations of MSA (30 ng/ml) and insulin (3000 ng/ml) was not more effective than either substance added alone, suggesting that both peptides act on the same mechanism(s) regulating beta-cell replication. Furthermore, an antibody to the insulin receptor did not prevent the stimulatory effects of either insulin or MSA on thymidine labeling of beta-cells. These results demonstrate that insulin can stimulate islet beta-cell replication directly, possibly through a receptor for MSA or another insulin-like growth factor.