Compensatory adrenal growth after unilateral adrenalectomy (ULA) leads to adrenocortical hyperplasia. Because zonal growth contributions are not clear, we characterized the phenotype of cortical cells that proliferate using immunofluorescence histochemistry and zone-specific cell counting. Rats underwent ULA, sham adrenalectomy (sham), or no surgery and were killed at 2 or 5 days. Adrenals were weighed and sections immunostained for Ki67 (proliferation), cytochrome P-450 aldosterone synthase (P450aldo, glomerulosa), and cytochrome P-450 11beta-hydroxylase (P45011beta, fasciculata). Unbiased stereology was used to count proliferating glomerulosa and fasciculata cells. Adrenal weight increased after ULA compared with sham and no surgery at both time points, and there was no difference between sham and no surgery. However, either ULA or sham increased Ki67-positive cells in the outer fasciculata at both time points compared with no surgery. Outer fasciculata-restricted proliferation is thus associated with adrenal weight gain in ULA but not sham. Experiment repetition using proliferating cell nuclear antigen and bromodeoxyuridine showed similar results. After ULA, adrenal DNA, RNA, and protein increased at both time points, whereas after sham, only adrenal DNA increased at 2 days. Compensatory growth thus results from hyperplasia and hypertrophy, whereas sham induces only a transient adrenal hyperplasia. Dexamethasone pretreatment prevented the increase in adrenal weight after ULA and blocked Ki67 labeling in the outer fasciculata but not zona glomerulosa in all groups. These results clearly show that the outer fasciculata is the primary adrenal zone responsible for compensatory growth, responding to steroid-suppressible stress signals that alone are ineffective in increasing adrenal mass.