To test whether there is a circadian rhythm in the ACTH response to stress, young female rats were exposed to a variety of ACTH-releasing stimuli at 0600 and 1800 h and changes in circulating ACTH and corticosterone were measured. The results of these experiments suggested that after the high intensity stimuli of laparotomy with intestinal traction or 250 μg histamine ip/100 g BW, the morning ACTH response was greater than the evening response. However, the ACTH response to ip saline was greater in the evening in one experiment and greater in the morning in a second experiment. Plasma corticosterone responses were faster and greater in the morning in the first experiment and in the evening in the second experiment. The ACTH response to 125 μg histamine ip/100 g BW was greater in the evening and the change in corticosterone was greater in the morning. Thus, after low intensity stimuli, the ACTH responses appeared to depend on both the lag time prior to the corticosterone response, and its magnitude. To test this possibility, rats were adrenalectomized and the ACTH response was measured 7.5 and 15 min after the start of surgery and after injection with either 2% EtOH-saline, or 50 μg corticosterone at operation, or with 30 μg corticosterone at 5 min. Compared with ACTH levels in rats treated with vehicle, ACTH levels were decreased 7.5 min after 50 μg corticosterone at operation (P < 0.01), but not after 30 μg corticosterone at 5 min. ACTH levels were slightly reduced 10 min after 30 μg corticosterone at 5 min compared with those of rats injected with vehicle at operation (P < 0.05). These results are consistent with the interpretation that corticosterone secretion modifies stress-induced ACTH secretion via the fast-feedback effect. Comparison of the ACTH responses to acute adrenalectomy plus injection with EtOH-saline at 0600 and 1800 h demonstrated that, in the absence of a corticosterone response to the stress, the ACTH response is greater in the morning than in the evening (P < 0.01). Finally, this grouP of experiments suggests strongly that the responsiveness of rat adrenal glands to ACTH increases markedly between 0600 and 1800 h.