In vitro and in vivo studies have suggested that the expression of the early response genes for Jun and Fos proteins plays an important role in adrenal cell proliferation. In order to study the expression pattern of the activating protein-1 (AP-1) family of oncogenes in the adrenal gland, we have used immunohistochemistry to localize Jun and Fos protein expression in rat adrenal cortex infused in situ with adrenocorticotropic hormone (ACTH), fibroblast growth factor 2 (FGF2), or both. The expression of AP-1 factors has been found to be correlated with in vivo ACTH and FGF2 proliferation in rats treated with dexamethasone and bromodeoxyuridine (BrdU). Induction of c-Jun and c-Fos in the zona fasciculata and of FosB in the zona reticularis suggests that, after ACTH stimulation, these proteins are the main AP-1 components in these zones. In vivo, ACTH increases BrdU-positive cells in the zona fasciculata and zona reticularis suggesting that the composition of AP-1 complexes in these zones is correlated with proliferation. Patterns of Fos and Jun induction by FGF2 do not resemble those after ACTH induction. However, in isolation, neither affects the zona glomerulosa. In the zona fasciculata, and more so in the zona reticularis, FGF2 modulates responses to ACTH, reducing the numbers of Jun-positive cells, Fos-positive cells, and DNA synthesis. This indicates that FGF2 antagonizes ACTH, and that ACTH thus controls the trophic effect independently of exogenous FGF2. Our results implicate the AP-1 family of transcription factors in the regulation of cell progression and the control of ACTH-induced proliferation in the zona fasciculata and zona reticularis.