Cells infected by tsA mutants of simian virus 40 (SV40) overproduce early RNA. Overproduction results from failure of the temperature-sensitive A protein (T antigen) to inhibit early transcription. The amount of early RNA in the cytoplasm, determined quantitatively from the kinetics of hybridization to labeled complementary SV40 DNA, was elevated at both permissive (32 degrees C) and nonpermissive (41 degrees C) temperatures in all the early mutants tested (tsA7, -30, -58, and -209), but not in the late mutant tsB4. The amount of early RNA in a culture maintained at 32 degrees C for 72 h and then shifted to 41 degrees C was maximum when each cell was infected initially with at least one plaque-forming unit of tsA58. Azidocytidine (2'-deoxy-2'-azidocytidine), which inhibits initiation of DNA synthesis, did not cause overproduction of early RNA in cells infected with wild-type SV40, showing that the effect seen with tsA mutants is not due to interference with initiation of DNA synthesis per se. In parallel infections at 41 degrees C, the amount of early RNA per copy of viral DNA was as much as 2,000 times greater with tsA58 than with wild-type SV40, even though there was no replication of the tsA58 DNA. Synthesis of late RNA could not be detected during the first 20 h of an infection by either virus at 32 degrees C, indicating that late and early transcription are under different control. In three cell lines transformed by tsA mutants, the amount of early RNA increased moderately after a shift from 32 to 41 degrees C, whereas with homologous cells transformed by wild-type virus, the amount of early RNA decreased, indicating that the A protein may be able to repress transcription of integrated SV40 DNA. All the observations are consistent with a simple model in which the binding of A protein at the origin of replication blocks either binding of RNA polymerase to the early promoter or its progress through the early gene(s).