The movement of horseradish peroxidase (HRP) in the epithelium of isolated rabbit iris/ciliary body preparations, has been studied with the electron microscope. HRP was applied at the stromal side of the epithelium, and was left for 60 and 120 min. The distribution pattern of HRP found in the epithelium of the iridial and ciliary processes is consistent with in vivo studies, i.e. the progression of HRP is blocked at the site of the zonula occludens of the superficial epithelium. The HRP distribution pattern found in the iris epithelium indicates that also the superficial epithelial cells of this epithelium are girdled by zonulae occludentes. Specimens treated with prostaglandins E1, E2, and F2alpha, showed no change in the epithelial distribution pattern of HRP, and the occluding zonules were found to be intact.