Angiotensin converting enzyme (ACE, E.C. 3.14.5.1) was localized in the rat eye by immunocytochemical staining using anti-rat lung ACE monoclonal antibody, and by autoradiography using the potent ACE inhibitor [3H]captopril. Highest levels of ACE immunoreactivity and [3H]captopril binding were found in the ciliary epithelium (Bmax = 1050 fmol [3H]captopril bound/mg protein) while lower levels were present in the sclera (Bmax = 500 fmol/mg protein). Scattered regions of the choroidal epithelium were weakly immunocytochemically stained by the anti-ACE antibody. No other major sites of labeling of ACE were demonstrated. In the ciliary epithelium, ACE was confined to cells bordering on the posterior chamber of the eye, similar to its presence at the ventricular surface of the choroid plexus of the brain. These findings suggest that ACE may metabolize a peptide involved in the regulation of aqueous humor production.