The expression of furin in pancreatic beta-cells induces faster cell growth and a decrease in differentiated beta-cell-specific characteristics. During the development of rat pancreatic islets, the prohormone convertases, PC2 and PC3, appear during late gestation and are expressed long after birth. We investigated the developmental change in another yeast Kex2 family endoprotease, furin, in rat islets in relation to islet cell growth. Furin had appeared by gestational day 18 and was distributed in islets as well as exocrine tissues. The expression of furin in islets increased toward the neonatal stage. Bromodeoxyuridine (BrdU) incorporation was also elevated in the perinatal period. On postnatal days 10 and 20, staining characteristics were attenuated. On day 25, immediately after weaning, furin staining began to localize in the beta-cell region, and staining in the alpha-cell region became fainter. On day 270, the staining in the alpha-cell region disappeared, and staining in the beta-cell region remained positive, but was weaker. We conclude that furin expression was greatest during the perinatal period, when BrdU incorporation into islets was maximal. Furin expression continued, however, even after the BrdU incorporation decreased. Thus, furin appears to control the proliferation as well as differentiation of islet cells.