To determine the interaction between ACTH and compensatory adrenal growth after unilateral adrenalectomy, we have studied the adrenals of young male rats treated with saline, ACTH, dexamethasone, or hypophysectomy and left or sham adrenalectomy (Adx or Sham). ACTH injected at the time of adrenal surgery inhibited the normal increase in [3H]thymidine incorporation into right adrenal DNA at 12 or 18 h (P < 0.05), but increased the RNA content. ACTH treatment (2, 4, or 8 U twice daily) in Shams caused a dose-dependent increase in adrenal weight and the RNA to DNA ratio at 24 h and, subsequently, a dose-dependent increase in adrenal DNA content at 72 h. ACTH treatment of Adx prevented the increase in adrenal DNA content at 24 h (P < 0.05), while increasing the RNA to DNA ratio (P < 0.05) in a dose-dependent manner. Treatment of rats with dexamethasone decreased adrenal weight and RNA content and attenuated the normal response to Adx. ACTH (1 U) at the time of adrenalectomy or sham operation in hypophysectomized rats abolished the normal difference between right adrenal weights in the two groups at 3 days. Adrenal growth occurred in rats hypophysectomized and adrenalectomized 24 h previously compared to Sham controls. These results show that adrenal growth mediated by ACTH is a different cellular process (hypertrophy) from that growth triggered by unilateral adrenalectomy (hyperplasia). Treatment with a long acting preparation of ACTH causes adrenals in rats from both the Sham and Adx groups to grow, and the process of ACTHmediated adrenal growth inhibits the rapid proliferative response to unilateral adrenalectomy. A proliferative response may occur by 3 days during treatment with large doses of long acting ACTH. Treatment at adrenalectomy with synthetic ±ACTH-(1 24) abolishes the normal increase in adrenal weight at 20 h.