A morphological and ultrastructural study is described which indicates that cellular activity in the adenohypophysis correlates well with the circulating levels of corticosterone. Intense secretory activity is observed in all tropic cells of the adenohypophysis over 10 days; thereafter the cellular morphology shows a return to the control condition. There are, however, differences in the degree of adaptation between the different tropic cells. After its initial hyperactivity, corticotrope activity returned to a control level by 20 days. Thyrotrope activity was also found to adapt to control activity, but only after 40 days. Similar patterns were observed in the somatotrope and gonadotrope, where the initial hypertrophy returned to control levels by 20 days; thereafter, however, an inhibition was observed. The luteotrope however, seems to be an exception in that its level of activity increased throughout the duration of the stress procedure.