We measured adrenal weights 3 days after left- or sham-adrenalectomy in male Sprague-Dawley rats at 10 ages between 10 and 155 days. Compared to sham-operated rats, the magnitude of the increase in right adrenal weight ranged between 19 and 32% after unilateral adrenalectomy and was significant at 10, 20, 40, 50, and 60 days (p < 0.01). Comparing left with right adrenals in unilaterally adrenalectomized rats, the right was heavier at all ages (p < 0.01); in animals 40 days and younger, this is due to normal adrenal growth rate (0.3 mg/day) as well as compensatory adrenal growth, while in older animals normal growth rate is slow (0.04 mg/day) and this difference in left and right adrenal weights is due primarily to compensatory growth. We conclude from this study that compensatory adrenal growth occurs 3 days after left adrenalectomy at all ages except 15 and 30 days. Resting levels of plasma ACTH and corticosterone did not differ between left- and sham-adrenalectomized rats at any age. The ACTH response to 1 min exposure to ether vapors was markedly reduced in 7- and 12-day-old rats (p < 0.01). We conclude that compensatory adrenal growth is a consistent response 3 days after unilateral adrenalectomy that depends neither on the age at which surgery is performed nor on the capacity of the rat to secrete markedly elevated levels of ACTH.