Although many hypotheses concerning cell proliferation and renewal in the adrenal cortex of mammals have been proposed, this topic has so far not been elucidated. Adrenocortical cells of adult mammals have low proliferative activity and take a considerable length of time to be renewed. This makes it difficult to investigate the dynamic features of their proliferation. To clarify the cell kinetics, we undertook a long term study in mice using an autoradiographic technique. We radiolabelled almost all the cells throughout the body in newborn mice with the exception of the neurons in central nervous system by the frequent subcutaneous injections of [3H] thymidine every 6 h for 30 d (pulse labelling). After this sequence of pulse labelling, we observed autoradiographically a decrease in the number of 3H-labelled cells in the adrenal cortex as a result of replacement with proliferated unlabelled cells (renewed cells). Single injections of [3H] thymidine (flash labelling) was also performed to examine DNA synthesis in the adrenal cortex. The investigations indicated that the adrenocortical cells proliferate at the border between the zona glomerulosa and the zona fasciculata, and that renewed cells which proliferated in that region move with time bidirectionally towards the cortical surface and the inner (medullary) surface. Half of the cortical cells in the zona glomerulosa, zona fasciculata and zona reticularis were replaced by renewed cells in 30, 60 and 120 d respectively. It took 200 d for almost all cortical cells to be replaced by renewed cells.