Human simian virus 80 (SV80) cells transformed by simian virus 40 (SV40) synthesize substantial quantities of the SV40 large T-antigen (Henderson & Livingston, 1974; Tjian, 1978) and cytoplasmic, poly(A)-containing RNA species that exhibit spliced structures characteristic of the SV40, early messenger RNA species that encode both large and small T-antigens (Flint & Beltz, 1979). When SV80 cells were infected with type C adenovirus, both the synthesis of SV40 large T-antigen and the appearance in the cytoplasm of newly synthesized, SV40-specific RNA sequences were inhibited during the late phase of infection. The results of hybridization to SV40 DNA of SV80 nuclear RNA, prepared from mock- or adenovirus-infected cells after labeling for short periods in vivo or in vitro, indicated that transcription of integrated SV40 was, by contrast, not disrupted during the late phase of adenovirus infection. Poly(A)-containing, nuclear RNA species that hybridized to SV40 DNA sequences and exhibited the sizes of spliced, large and small T-antigen mRNA species were also synthesized in infected cells at a time when the corresponding mRNA sequences did not leave the nucleus. These results suggest that the failure of non-adenoviral mRNA sequences to enter the cytoplasm of adenovirus-infected cells does not reflect inhibition of either their transcription or the normal enzymatic processing reactions to which pre-mRNA species are subject. Several lines of evidence do, however, establish that nuclear, SV40-specific RNA sequences are less stable in adenovirus-infected compared to mock-infected SV80 cells.