Diurnal rhythms of plasma ACTH and plasma corticosterone concentrations were studied in intact rats at 1, 3, 5, and 7 weeks of age and in bilaterally adrenalectomized adult rats 1, 3, 5, and 7 weeks after transplantation of the neonatal (day 1) adrenal to the anterior chamber of the eye. Whereas there was no difference in adrenal weight or plasma corticosterone concentration between intact and transplanted rats during adrenal development, the plasma ACTH concentration was significantly increased in transplanted rats relative to that in intact rats throughout development. A diurnal rhythm of plasma corticosterone concentration was observed in transplanted rats 5 and 7 weeks after transplantation; however, the onset of the rhythmicity was delayed relative to that of intact rats, who displayed rhythmicity at 3, 5, and 7 weeks of age. Elevated plasma ACTH concentrations in chronically cannulated transplanted rats exhibited diurnal variation in parallel with plasma corticosterone concentrations. Adult rats bearing adult adrenal transplants under the kidney capsule for 5 weeks had plasma ACTH concentrations in the evening that were not different from those in unilaterally adrenalectomized, sham-transplanted rats. In contrast, adult rats bearing neonatal adrenal transplants under the kidney capsule for 5 weeks had increased plasma ACTH concentrations in the evening. Thus, the increased plasma ACTH concentration after adrenal transplantation is dependent on the age of the transplant and not on the site of transplantation. The finding of elevated plasma ACTH concentration associated with normal plasma corticosterone concentration suggests that adrenal responsiveness to ACTH is decreased after transplantation of the neonatal adrenal to the adult rat.