Cycloheximide, at a concentration of 10 ,ug/ml, rapidly blocked protein synthesis in L cells infected with reovirus. When the drug was added before 5 hr postinfection, synthesis of both single-and double-stranded varieties of virus-specific ribonucleic acid (RNA), which normally commences between 6 and 7 hr after infection, was blocked. When the cycloheximide was added at 9 hr after infection, uptake of uridine-H3 into RNA, for the succeeding 6 hr at least, was similar to that of an infected

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... ture without the drug. This latter uptake of uridine-H3 in the presence of cycloheximide was largely into single-stranded RNA, since doublestranded RNA synthesis was rapidly and markedly inhibited by the cycloheximide. Single-stranded RNA formed in the presence of cycloheximide was found not to be a precursor of viral progeny, double-stranded RNA. Synthesis of doublestranded RNA in the infected cell probably requires prior synthesis of a new protein, which has a rapid rate of turnover. The possibility that formation of single-stranded RNA is preceded by synthesis of a second new protein is discussed.