From 15 to 20% of the RNA of virulent strains of poliovirus is degraded at 35 °C several hours after maximal synthesis is reached. From 20 to 30% of the RNA of attenuated viruses is degraded under the same conditions. Cultures placed at 40 °C in the presence of 3 mM guanidine during the linear stage of RNA synthesis lead to breakdown of 15 to 40% of the RNA of virulent viruses and 30 to 40% of the RNA of attenuated viruses. It is doubtful that breakdown of RNA alone can explain the temperature sensitivity of attenuated viruses.