The inhibition of herpes simplex virus type 1 plaque formation by acyclovir, bromovinyldeoxyuridine, 9-(3,4-dihydroxybutyl)guanine, and 9-(4-hydroxybutyl)guanine at different thymidine concentrations was analyzed in Lineweaver-Burk plots. Linear competitive patterns between thymidine and the nucleoside analogs were observed for the inhibition of herpes simplex virus type 1 plaque formation. A new constant, the reversal constant Kr, was introduced to describe the sensitivity in cell culture of an antiviral drug to the reversal of its viral activity by a metabolite (e.g., thymidine).