Bromodeoxyuridine (BrdU) was administered by a single intraperitoneal injection to immature (14 days) male and female and adult male Sprague‐Dawley rats. Animals were killed at intervals from 2 hr to 28 days following injection. Labelled cells in the adrenal cortex were identified by an indirect immunoperoxidase technique using a monoclonal antibody to BrdU. At 2 hr, labelling was maximal in the outer zona fasciculata and zona glomerulosa in both prepubertal and adult rats. The numbers of immunopositive cells were greater in the 14 day rats. In both groups, the front of immunopositive cells moved deeper into the cortex with time. These results support the centripetal migration theory of adrenal growth.