The relationship of the intracellular events leading to the production of polyoma pseudovirions in primary mouse embryo cells has been investigated. Replication of polyoma deoxyribonucleic acid (DNA) began 18 hr after infection. Assembly of viral capsid protein occurred 12 hr later. Intracellular fragments of host cell DNA, of the size found in pseudovirions, were first detected 36 hr after infection. The amount of intracellular 14S host DNA that was produced during infection was seven times greater than the amount of polyoma DNA synthesized. The relative pool sizes of polyoma DNA and 14S DNA at the time of virus assembly may dictate the amounts of polyoma virus and pseudovirus produced.