Herpes Simplex Encephalitis-Evans et al. account of the sytemic use of idoxuridine in the treatment of a severe D.N.A.-virus infection, herpes simplex encephalitis.The place of idoxuridine in the chemotherapy of viral encephalitis, with or without corticosteroids, must await further trial; at least the facts of this case suggest that it may have antiviral action within the central nervous system.As the chemotherapy of virus disease. advances drugs are likely to be developed which are type-specific. Early identification of virus will be important. Brain biopsy, with the technique of Blackwood and Cumings (1959), is a safe and reliable procedure, but successful isolation of virus may depend on the speed with which material reaches the laboratory.ADDENDUM.-Since this report was submitted for publication Breeden et al. (1966) have described a case of herpes simplex encephalitis in a middle-aged man who recovered after treatment by cerebral decompression and a seven-day course of idoxuridine.