The aim of this experiment was to examine, using autoradiographic techniques, proliferative activity in the adrenal cortex of rats on day 20 of fetal life, and to follow the fate of labelled cells 48, 168, 336 and 672 h after injection of [ ³ H]thymidine. On day 20 of fetal life, labelled cells were more numerous in the zona glomerulosa than in the other zones. The fate of the labelled cells in each cell compartment showed centripetal migration, but their displacement from day 20 of pregnancy up to 5 days post partum , combined with a reduction in the size of the adrenal gland, suggested a real migration. The centripetal movement was found in the cortex when growth had effectively ceased.