Polyoma virus induced in quiescent, Go-arrested mouse kidney cells a lytic infection. Synthesis of the polyoma T-antigens began 7–8 h after infection and was followed by a mitotic reaction of the host cell comprising stimulated synthesis and accumulation of cellular (mainly ribosomal) RNA and protein and duplication of the host cell chromatin (S–phase). In the present work we focused attention on nucleoplasmic transcription, i.e. synthesis of hnRNA, 5S RNA and tRNA. To inhibit selectively nucleolar transcription we used low concentrations of actinomycin D (act. D). Synthesis of 45S precursor- ribosomal RNA in mock- and polyoma-infected mouse kidney cells was completely blocked by 0.05 μg/ml act.D within 2 h. In mock-infected cells also nucleoplasmic transcription was rather sensitive against 0.05 μg/ml act.D. Polyoma-induced stimulation of nucleoplasmic transcription began around 12 h and was paralleled by the development of resistance against act. D. Resistance of nucleoplasmic transcription in virus-infected cells was thus similar to that observed by others in uninfected, proliferating mammalian cells. The possible biological implications of these results are discussed.