SUMMARY: Experiments were carried out to discover whether oestrogen modifies the effect of ACTH on the adrenal glands of the female rat. It was found that oestradiol dipropionate (ODP) did not alter the response to exogenous ACTH in either spayed or control-operated animals. Compensatory hypertrophy of the remaining gland after unilateral adrenalectomy was increased by treatment with ODP in ovariectomized rats, but was unaffected in control-operated animals. In hypophysectomized rats the adrenal glands were significantly heavier after treatment with ACTH and ODP then they were when ACTH was given alone. The oestrogen alone had no effect in hypophysectomized animals.