Angiotensin-converting enzyme (ACE) was localized by immunocytochemical methods in the rat adrenal gland. The three-layer peroxidase-antiperoxidase method was used with light and electron microscopy. At the cortical level, ACE immunoreactivity occurred only on the luminal side of vascular endothelial cell membranes in the adrenal capsule, whereas no ACE activity was found in the zona glomerulosa, fasciculata, or reticularis. ACE immunoreactivity was found mainly but not exclusively on the luminal side of all the types of vessels in the adrenal medulla such as capillaries, venous sinuses, and arteriae medullae. In addition, ACE was localized on the plasma membrane of chromaffin cells but never in Schwann cells nor in nerve fibers. These results provide evidence for a local production of angiotensin II at the vascular level in the adrenal gland. In addition, the presence of ACE on the plasma membrane of chromaffin cells suggests that angiotensin II or other peptides, such as bradykinin, could be metabolized here.