The pyrimidine metabolism of fibroblasts infected with herpes simplex virus type 1 was studied. Herpes simplex virus type 1 infection increased the dTTP pool and thymidylate synthetase activity but reduced thymidine excretion. Addition of acyclovir to infected cells increased thymidine excretion, the dTTP pool, and thymidylate synthetase activity. Addition of a virus-specific ribonucleotide reductase inhibitor (A723U) decreased all three. The synergy between the two compounds is discussed.