Hybridization probes for two nuclear-coded chloroplast proteins of Lemna gibba L. G-3 have been constructed in order to investigate phytochrome regulation of specific sequences. The first probe is a cDNA clone encoding the small subunit of ribulose 1,5-bisphosphate carboxylase. This probe was isolated from a set of Lemna cDNA clones in the bacterial plasmid pBR322. The second probe is a subclone of a genomic clone encoding the light-harvesting chlorophyll a/b-protein. This clone was isolated from a set of genomic clones constructed in the lambda vector Charon 4 with L. gibba DNA fragments generated by partial EcoR1 digestion. The identity of these clones was confirmed by in vitro translation of RNA which hybridized to the cloned DNA. Plants grown under continuous white light contain high concentrations of both RNA sequences; however, when these plants are put into darkness the concentration of these RNAs decreases rapidly relative to the total amount of RNA. Plants grown in the dark with intermittent red light (2 minutes/8 hours) and put into complete darkness for 8 days also contain lower concentrations of the sequences in the total RNA. One minute of red light after this dark period results in a rapid increase in the levels of RNA hybridizing to the probes. The effect of red light can be reversed by far-red light. These experiments demonstrate that phytochrome action can rapidly influence either the rates of transcription or the rates of degradation of these mRNAs.