The phosphonylmethoxyalkyl derivatives (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [(S)-HPMPA], 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP) were evaluated for their in vivo efficacies in several animal model infections, i.e., mice infected intravenously with vaccinia virus and mice infected intracutaneously, intraperitoneally, or intracerebrally with herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) or thymidine kinase-deficient (TK-) HSV-1. (S)-HPMPA inhibited the development of tail lesions caused by vaccinia virus if it was administered intraperitoneally or subcutaneously at a dosage as low as 5 mg/kg per day. All three compounds completely suppressed the development of skin lesions and the mortality associated therewith in hairless or athymic nude mice inoculated intracutaneously with HSV-1 or TK- HSV-1, if they were administered topically at a concentration as low as 0.1%; when (S)-HPMPA was applied topically at a concentration of greater than or equal to 0.3%, it completely abrogated mortality resulting from intracutaneous HSV-2 infection. Most dramatic were the effects shown by the compounds in mice inoculated intracerebrally with HSV-1, HSV-2, or TK- HSV-1, in which all three compounds given intraperitoneally at a dose of 50 or 100 mg/kg per day effected a significant reduction in the mortality rate of HSV-1-infected mice. The mortality of mice infected intracerebrally with HSV-2 or TK- HSV-1 was significantly reduced even when (S)-HPMPA was given at doses as low as 10 mg/kg per day. These data point to the great potential of the phosphonylmethoxyalkylpurines for both topical and parenteral treatment of HSV-1, HSV-2, and TK- HSV-1 infections.