We used two sequential 7.5 ml/kg hemorrhages, spaced 24 h apart, in the chronically prepared, pentobarbital-anesthetized dog to study the effects of repeated stimuli on the adrenocortical system. Adrenal secretion of cortisol, peripheral cortisol, and ACTH were measured. All three variables increased after an initial 7.5 ml/kg hemorrhage. When the hemorrhage was repeated 24 h later, the secretory response of cortisol began more rapidly (by 4 min), reached a higher peak, and was more prolonged than the response on day 1. ACTH rose to significantly higher values than on day 1, but only after 8 min. There were no differences in cardiovascular variables after hemorrhage on the 2 days. A repeated 3.75 ml/kg hemorrhage did not lead to a potentiated response. These results confirm reports that after physiological stimulation, changes occur in the pituitaryadrenal system that may lead to a potentiated response to later stimuli. The mechanism of these changes is unknown, but our results suggest that both an increase in circulating ACTH and a change in adrenal sensitivity to ACTH may be involved.