The effect of cycloheximide on RNA metabolism in tobacco leaf discs was analyzed to signify its inhibitive or stimulative effect on TMV multiplication. RNA content increased in the leaf discs incubated in 1.0 or 3.0 ppm solution. 32P-incorporation into RNA, however, was accelerated at 1.0 ppm and disturbed at 3.0 ppm, suggestirg th~t the increment of RNA in the leaf disc<- treated at 1. O ppm is qualitatively different from that in the discs incubated at 3.0 ppm, i. e., the former is the result of stimulated synthesis while the latter is mainly due to the inhibition of degradation. Column chromatograms with methylated albumin Kiselguhr clearly elucidated that incubation of discs in 3.0 ppm led to the abolition of the integrity of the light ribosomal RNA. The change in chromatographic pattern was discernible 24 hr after the initiation of treatment of leaf discs. Incubation in 1.0 ppm cycloheximide, however, did not give rise to a drastic change in chromatograms, although an undulate incorporation of 32P-orthophosphate into ribosomal RNAs was noticed. 14C-glycine incorporation into RNA suggested that cycloheximide not only enhances RNA synthesis but also stimulates the synthesis of purine bases. Ribonuclease activity of leaf homogenate remarkably decreased by cycloheximide treatment, suggesting that the accumulation of RNA in the discs treated at 3.0 ppm could be acounted for by the retardation in RNA degradation. The inhibitory and stimulative effects of cycloheximide on TMV multiplication were discussed on the basis of these results.