Abstract.Three different substances, [14C]inulin, [14C]polyethylenglycol (PEG, molecular weight 4000) and [3H]sucrose have been examined for suitability as markers for determination of the extracellular volume (ECV) in the bovine corneal epithelium and stroma. In both tissues, sucrose was the best marker reaching equilibrium of distribution within 90 min. Inulin and 4000 PEG have been found to be unacceptable because of a slow rate of penetration of the ECV. In addition, both inulin and 4000 PEG gave considerably lower values for the ECV than sucrose. Using the value for ECV determined with sucrose as a marker in the bovine corneal epithelium, i.e. 21.6 ± 1.4% of tissue water, and the results obtained previously by the chemical analysis of this tissue (Midelfart 1987) the intracellular concentrations of 34.4 ± 8.6 mmol Na/kg H2O and 154.3 ± 11.1 mmol K/kg H2O were determined. These results were compared with the results obtained by other methods in corneae from other speci