The presence of constituents of the renin-angiotensin system (RAS) in ocular tissues and fluids suggests this system is involved in ocular physiology. Angiotensin II (AngII) is the main biological effector of the system, so we measured AngII in plasma and in aqueous humor of the anterior ocular chamber of patients undergoing cataract extraction. Untreated normotensive patients were compared with arterial hypertensive patients taking either diuretics which stimulate the RAS or angiotensin converting enzyme (ACE) inhibitors which reduce the production of AngII. Plasma levels of AngII were higher in patients on diuretics (5.46 +/- 1.04 fmol/ml; mean +/- SEM) than in untreated cataract patients (2.28 +/- 0.32 fmol/ml, p < 0.02), and were very low with ACE inhibitors (0.51 +/- 0.18 fmol/ml). In aqueous humor, AngII was measurable in 7 of 11 patients on diuretics (median 1.1 fmol/ml), and in 6 of 16 normotensive patients (median < 0.55 fmol/ml), but not in aqueous humor of 4 patients receiving enalapril or captopril. These results demonstrate the presence of AngII in the eye but do not exclude either its sequestration in the eye or local production. The possibility of individual measurements of intraocular AngII will permit more precise determination of its role in future studies.