The study of the permeability of the cornea to 5-iodo-2′-deoxycytidine (IDC), an antiherpetic agent was performed-in the rabbit. In a first experiment, using125I-IDC eye-drops and a sustained contact between the drug and the cornea, we showed that the penetration of IDC in the aqueous humor was important. In a second experiment, using a HPLC method, we studied comparative ocular penetration and metabolism of IDC and 5-iodo-2′-deoxyuridine (IDU).