Herpes simplex virus can confer to thymidine kinaseless cells the ability to incorporate exogenously supplied thymidine into acid precipitable material. However no incorporation of exogenously supplied deoxycytidine into acid precipitable material can be detected after infection of deoxycytidine kinaseless cells by herpes simplex virus. This failure to incorporate exogenous deoxycytidine is not due to the failure of the deoxycytidine phosphorylating activity of the virus induced deoxycytidine kinase but to a block in the metabolism of deoxycytidine monophosphate in herpes simplex virus infected cells. This block becomes evident with the appearance of the virus induced deoxypyrimidine kinase activity.