Cholinergic supersensitivity of the iris sphincter helps to make the diagnosis of "tonic pupil." Because uncertain responses sometimes occur with mecholyl 2.5%, dilute solutions of pilocarpine are often used. There has always been some question as to whether individual variation in corneal permeability was one of the factors contributing to the variability in pupillary constriction to topical cholinergics in these patients. In this study, the degree of intraocular penetration of topical fluorescein was compared with the degree of pilocarpine-induced miosis in six tonic pupils and in seven age-matched normal controls. A noncontact fluorophotometer and a computerized videopupillogram were used. The patients showed a significant cholinergic supersensitivity in their affected tonic pupils and had normal intraocular penetration of fluorescein through the cornea in both eyes. It is concluded that the cholinergic supersensitivity of the iris in tonic pupils is not the result of increased corneal permeability but instead results from an increased sensitivity of the cholinergic receptors in iris sphincter muscles.