Quantitative structure-activity relationships of the Hansch-type were developed to account for inhibition of thymidine kinases from Herpes simplex viruses types 1 and 2 (HSV1,2) by N2-phenylguanines. Derivatives with meta and/or para substituents on the phenyl ring display a wide range of overlapping, but not identical, potencies as inhibitors of the enzymes. IC50 values for 36 (HSV1) and 35 inhibitors (HSV2) were used to develop equations using hydrophobic (pi), electronic (sigma, R), and group size (MR) parameters. Equations 1 and 2 with correlation coefficients of 0.797 and 0.805, respectively, were obtained for inhibitors of the types 1 and 2 enzymes. Potencies were correlated positively with pi values of meta substituents but negatively with pi values of para substituents in the phenyl ring. Positive correlations were also obtained with the resonance parameter R of para substituents and with sigma constants of meta substituents. The most potent inhibitor of both enzymes was N2-[m-(trifluoromethyl)phenyl]guanine, although HSV2 thymidine kinase was more sensitive to certain compounds than the HSV1 enzyme.