Deoxyribonucleoside triphosphate pools were analysed in both exponentially growing and serum starved wild type BHK C13 cells and in a derivative of this cell line which lacks both thymidine kinase and deoxycytidine kinase activities, before and after infection with herpes simplex virus. Serum starved BHK cells had low levels of all four deoxyribonucleoside triphosphates. In exponentially growing cells all pools were expanded, the pool of dCTP being largest and dGTP the smallest. The dATP and dTTP pools were of intermediate sizes. In exponentially growing deoxypyrimidine kinase free cells the pools, with respect to level and distribution, were the same as those observed in wild type cells. After infection with herpes simplex virus there were marked changes in the levels of all deoxyribonucleoside triphosphate pools; the most predominant being a 25- to 50-fold expansion of dTTP pool. The pools of dCTP and dGTP also increased while the pool of dATP was very much reduced. These effects could be observed in both wild type and mutant cells.