Incubation of tobacco mosaic virus (TMV)-infected [Nicotiana tabacum] tissue at 40.degree. C for 12 h destroyed the capacity for TMV RNA replication. Upon shift of the tissue from 40.degree. to 25.degree. C, synthesis of TMV RNA did not resume. Incubation at 40.degree. C for 1 h did not destroy the TMV RNA replicase, because viral RNA synthesis resumed upon return to 25.degree. C. Upon further incubation at 25.degree. C, the synthesis rate of TMV RNA gradually declined. Upon still further incubation at 25.degree. C (16-20 h), the synthesis of TMV RNA in tissue incubated at 40.degree. C for 1 or 2 h recovered. This recovery was inhibited by cycloheximide but not by 2-thiouracil.