Proliferative activity of the adrenal cortex was studied following injections of thymidine-H3. Normal male Long-Evans rats had less activity than comparable animals of the Sprague-Dawley strain. Of the Long-Evans rats killed at different hours of the day, those injected with thymidine-H3 at 6 P.M. and killed six hours later had the greatest activity. Normal male rats 35 to 73 days of age had almost all activity in the glomerulosa and outer fasciculata of the cortex. This activity was greatly increased two or three days after injecting colchicine, compound 48/80, or carbon tetrachloride. Such stressing agents did not cause any increase in activity during the first 24 hours. Three days after carbon tetrachloride a considerable increase in proliferative activity occurred in the inner zona fasciculata and reticularis of the adrenal of female rats during diestrus. Male rats killed 28 to 40 days after injecting thymidine-H3 showed no evidence of centripetal migration of cells into the reticularis during that time. The absence of centripetal migration and the marked responsiveness of the inner fasciculata and reticularis of female animals after stressing agents supports the zonal theory of function in the adrenal cortex.