The effect of blue and red light on the synthesis of transfer and 5S ribosomal RNA in autotrophic cultures of Chlorella pyrenoidosa was studied by pulse labeling experiments with tritiated guanosine. Compared with darkness or red light (679 nm), blue light (457 nm) of low intensities (quantum flux: 0.5-5×10(-10) mol photons cm(-2) s(-1)) stimulated incorporation of guanosine into transfer and 5S ribosomal RNA within the first 5 min of illumination. This blue light effect was abolished by cycloheximide, an inhibitor of protein synthesis on 80S ribosomes, but not by rifampicin, an inhibitor of chloroplastic transcription, nor by lincomycin, an inhibitor of chloroplastic translation. The rifampicin-insensitive synthesis of transfer and 5S ribosomal RNA was nuclear transcription as shown by RNA-DNA hybridization. The blue light effect on nuclear RNA synthesis was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea, an inhibitor of photosynthesis electron transport. Further evidence for a photosynthesis-independent photocontrol of RNA synthesis was provided by experiments with the colorless mutant 125a of Chlorella vulgaris. Blue light stimulated incorporation of guanosine into cytoplasmic 25S and 18S ribosomal RNA as well as into transfer and 5S ribosomal RNA, whereas incorporation in red light was the same as that of the dark control.