A series of N2-phenylguanines was synthesized and tested for inhibition of the thymidine kinases encoded by Herpes simplex viruses type 1 and type 2. Compounds with hydrophobic, electron-attracting groups in the meta position of the phenyl ring such as m-trifluoromethyl (m-CF3PG, IC50 = 0.1 microM) were the most potent inhibitors of both enzymes. Many derivatives were significantly more potent against the type 2 thymidine kinase, and can effectively discriminate between the two enzymes. Among other N2-substituted guanines, alkyl and benzyl derivatives were moderately potent inhibitors, and the type 2 enzyme was again more sensitive than the type 1 enzyme. None of the compounds inhibited the thymidine kinase isolated from the host HeLa cell line, suggesting that members of this class of compounds may be useful nonsubstrate, antiviral compounds for latent herpesvirus infections.