The distribution of intravenously injected horseradish peroxidase in the iris of prostaglandin (E1 and E2)-treated rabbit eyes, has been studied with the electron microscope. Peroxidase was demonstrated in the stroma of all parts of the iris. The tracer was found throughout interendothelial clefts of iris vessels, indicating that these vessels had become permeable to peroxidase. The distribution of peroxidase in the iris epithelium indicates that the posteroir epithelial cells are girdled by zonulae occludentes.