OBJECTIVE To simultaneously determine the tear turn over rate (TT) and corneal epithelial permeability (Pdc) in normal subjects and patients with KCS by a single-drop fluorophotometric technique using disodium fluorescein (DSF) or carboxyfluorescein (CF). METHODS DSF was instilled in one eye chosen at random and CF in the fellow eye of 13 normal subjects and in 13 patients with KCS. TT and Pdc were determined using a single-drop technique on a Fluorotron Master. RESULTS In normals and KCS subjects, TT was found to be independent of age and sex and essentially identical for DSF and CF. TT was approximately 42% lower in KCS subjects than normals (Table). TT was independent of Schirmer's I and Pdc. [table: see text] Pdc values were similar for DSF and CF in normals and increased with age. In KCS subjects, Pdc was 3-4 times higher compared to normal subjects (p < 0.01) and was directly correlated with the severity of corneal punctate staining. Pdc was independent of TT. CONCLUSIONS Patients with KCS are more susceptible to the therapeutic and toxic effects of topical medications and preservatives due to increased corneal epithelial permeability. With the decreased TT in KCS patients, there is also slower elimination of substances from the tear film. This, combined with the increase in epithelial permeability, puts the KCS eye doubly at risk for the toxic effects of topically applied substances.