Circadian rhythm of blood corticosterone in adrenalectomized rats with (AT rat) or without adrenal auto-transplants was investigated. Blood samples were obtained individually at 4-hour intervals for a 24-hour period per week for 5–7 consecutive weeks by the tail tip incision method. Corticosterone responses of the autotransplanted adrenal gland to ACTH gradually increased with time after the implantation, and by 5 weeks had attained a level comparable to that of intact rats. Concurrently with this change, circadian adrenocortical rhythms in AT rats became evident and they were able to respond to a reversed photoperiodic environment. The effects of constant illumination or food restriction on the corticosterone rhythm in AT rats were indistinguishable from those in intact rats in every respect. A sharp drop and disappearance of circadian periodicity were observed in the blood corticosterone levels by removal of autotransplanted adrenals. In some of the adrenalectomized rats, the daily rhythm of the blood corticosterone levels reappeared 5–7 weeks after surgery owing to the hypertrophy of accessory adrenal glands or regeneration from capsular fragment left in situ. These results suggest that neural input to the adrenal is not essential for the occurrence of circadian adrenocortical rhythms.