Uptake of [3H]thymidine by serum-starved baby hamster kidney cells infected with herpes simplex virus type 2 was investigated by light microscopic and electron microscopic autoradiography. The distribution of incorporated label altered throughout the period of viral DNA synthesis. It was restricted initially to a few well-defined sites within the nucleus which increased in size and spread as infection proceeded until the entire nucleus was involved. The label was not associated with any identifiable subnuclear structure although there was a high degree of association with the nuclear membrane at early times post-infection.