Experimental herpesvirus encephalitis in weanling mice was treated with either cytosine arabinoside or adenine arabinoside to determine the comparative effectiveness of the two compounds on survival and on the concentration of virus in the brain. The uniformly fatal course of the encephalitis was not altered by any dosage of cytosine arabinoside. In contrast, treatment with adenine arabinoside resulted in long-term survival of the majority of the infected animals. The concentration of virus measured in the brains of animals treated with two different dosages of cytosine arabinoside indicated initial suppression of viral replication with a subsequent rise to levels higher than those in the untreated controls. In the brains of adenine arabinoside-treated animals, titers of virus gradually diminished to undetectable levels by the eighth day after institution of therapy.