Using in situ end-labelling of DNA breaks (TUNEL) and staining with propidium iodide, we have investigated the occurrence of apoptosis in the adrenal cortex of hypophysectomized rats. Apoptosis was detected in the zona fasciculata and reticularis 3 days after hypophysectomy. Six days after hypophysectomy rats showed apoptotic cells mainly in the zona fasciculata. With the exception of the zona reticularis, cell death was largely prevented by ACTH replacement in 3-day hypophysectomized animals. The present results demonstrate that ACTH can reduce but not fully prevent apoptosis in the adrenal cortex of hypophysectomized rats and suggest that lack of other factors, such as other pituitary hormones, may be involved in the onset of apoptosis in the zona reticularis.