PURPOSE: Membrane-associated carbonic anhydrase (CA) activity is probably of great importance for transepithelial transport of ions and fluid. Histochemical studies have indicated its presence in the eye, but such histochemical data are difficult to evaluate because of interference from cytoplasmic CA isozymes, of which CA II is predominant. CA II-deficient mice offered the possibility to study the localization of membrane-associated CA activity, without influence from CA II: METHODS: The localization of CA in the eyes of CA II-deficient mice and of normal mice was studied by the cobalt-phosphate histochemical method. RESULTS: In both types of mice, intense histochemical CA activity was associated with the apical and basolateral membranes of the pigmented and nonpigmented ciliary epithelium, of the corneal endothelium, and of the pigmented epithelium of the retina. It also was localized at the cell borders of the Müller cells and of the lens epithelium and fibers. There also was CA activity in the endothelium of the capillaries of the choroid and retina but not in that of the larger vessels. CONCLUSIONS: Membrane-associated CA activity is found in many ocular cells known to transport fluid and ions. Inhibition of the CA activity of the basolateral membranes of the ciliary nonpigmented epithelium probably explains the reduction of aqueous humor flow seen after the administration of CA inhibitors.