Certain inhibitors of normal deoxyribonucleic acid (DNA) synthesis have been found to be clinically effective antiviral agents. After the observation that idoxuridine (5-iodo-2′-deoxyuridine, IUdR, IDU) inhibited herpes simplex virus plaques in a cell culture system (Herrmann, 1961),1 it was reported that this agent was highly active in curing herpes simplex keratitis in rabbits (Kaufman, 1962)2 and in man (Kaufman et al, 1962).3 Subsequently, another pyrimidine nucleoside, 1-β-d-arabinofuranosylcytosine hydrochloride (cytosine arabinoside or CA) was shown to cure herpes keratitis in rabbits (Underwood, 1962)4 and in man (Kaufman and Maloney, 1963).5 Because of the potential importance of these agents in antiviral chemotherapy, we continued our work on CA and expanded our studies to include other pyrimidine arabinosides. Materials and Methods In Vivo. —Previously described procedures (Underwood, 1962)4 used to produce experimental herpes simplex keratitis in rabbits were modified as follows. Two drops of 2%