Barley (Hordeum vulgare) leaf segments unroll and green when illuminated. Illuminated segments also have an increased capacity for RNA synthesis. Part of this increased RNA synthesis may be attributed to an increased RNA polymerase activity. In addition, following illumination there is an increased formation of polysomes.Analyses of RNA synthesized during illumination showed that the radioactive RNA which accumulated was predominantly associated with cytoplasmic ribosomal RNAs. It appears that the early phases of greening are achieved without chloroplast RNA synthesis. Following extended illumination there was evidence of chloroplast ribosomal RNA synthesis; however, this occurred after appreciable chlorophyll synthesis. Actinomycin D, which effectively inhibits RNA synthesis and leaf unrolling, restricts chlorophyll synthesis only during the later stages of illumination. 5-Fluorouracil inhibits the bulk of RNA synthesis but not greening, unrolling, or polysome formation. Studies with inhibitors of protein synthesis have demonstrated a requirement for protein synthesis concomitant with chlorophyll production and leaf unrolling.