Acyclovir is an antiviral agent which is highly active in vitro against Herpes simplex (HSV) types I and II and Varicella zoster viruses, but its toxicity to mammalian cells is low. Acyclovir is phosphorylated to the active compound Acyclovir triphosphate after entry into herpes infected cell. The _first step in this process requires the presence of the viral-coded thymidine kinase. Acyclovir triphosphate acts as an inhibitor of and substrate for the herpes specifi_ed DNA polymerase preventing further viral DNA synthesis without affecting normal cellular processes.