Summary Young, maturing and aged tobacco leaves were infected with tobacco mosaic virus (TMV) strain vulgare or flavum. TMV-RNA synthesis and accumulation were followed. Flavum and vulgare RNAs had different patterns of synthesis and accumulation: flavum-RNA may be unstable. In healthy leaves, ribosomal RNA synthesis (measured by [32P] incorporation) increased to a peak before the leaf reached its maximum length then declined to 25% of the maximum as the leaf aged. TMV infection of a young leaf caused immediate and persistent inhibition of ribosomal RNA synthesis. Ribosomal RNA synthesis in older leaves showed three phases after TMV infection. (1) One day after inoculation, ribosomal RNA synthesis was higher than in healthy leaves. (2) During the main accumulation of TMV-RNA, ribosomal RNA synthesis was inhibited. (3) Following TMV-RNA accumulation, ribosomal RNA synthesis rose, often to levels higher than in healthy leaves. The half-life of ribosomal RNA in a TMV-infected leaf was found to be twice that in a healthy leaf. These observations are discussed in relation to leaf development and virus multiplication.