Potassium iodide (KI) passes the cornea of isolated rabbit eyes with kinetics of approximately 0.25 mumol/h/ml aqueous humor. In photodynamic reactions, simulated as light-dependent decay of S-methyl-alpha-ketobutyric acid in the presence of riboflavin, KI acts as an antioxidant cooperating with internal scavengers such as ascorbate. With the simple model reactions applied it may be possible to study mechanism and functions in vivo of eye-protecting factors or combinations of compounds.