To determine the requirements for gene expression in mammalian germ cells, circular double-stranded simian virus 40 (SV40) DNA molecules containing deletions in sequences controlling transcription and replication were injected into the nucleus of mouse oocytes. Expression of large (T-Ag) and small (t-Ag) tumor antigens ("early gene products") required at least three GGGCGG boxes, but did not require either the origin of viral DNA replication (ori) or a TATA box. Expression of capsid antigen VP1 ("late gene products") required at least three GGGCGG boxes, sequences between nucleotides 197 and 273 in the 72-bp repeat region, and transactivation by T-Ag. These results are consistent with the requirements for expression of the same genes in differentiated mammalian cells. Surprisingly, however, the 72-bp repeats ("enhancer elements") that are required for expression of T-Ag and t-Ag genes in differentiated cells were not required in mouse oocytes. Similarly, expression of both the early and late genes was unaffected in mouse oocytes by the absence of either DNA replication or an intact ori sequence, components required for maximum expression of late genes in differentiated cells. Thus, mammalian oocytes effectively utilize promoters that are fully active in mammalian differentiated cells only when associated with either enhancer elements or DNA replication. Furthermore, requirements for expression of SV40 genes in mouse oocytes are distinctly different from those reported for Xenopus oocytes. This suggests that caution should be exercised when extrapolating conclusions drawn from experiments with amphibian germ cells to mammalian germ cells.