The light-induced greening of etiolated barley plants is used as a model to study the light-dependent control of plastid development. Upon illumination a rapid transformation of etioplasts to chloroplasts is induced. The effect of illumination does not only include the light-dependent chlorophyll synthesis but also the appearance or dedine of specific proteins within the plastid membrane fractions. So far two of these proteins have been studied in detail. The light-harvesting chlorophyll a/b protein (LHCP) is one of the major protein constituents of the thylakoid membrane of chloroplasts. However, this protein is not detectable among the membrane polypeptides of etioplasts. Illumination of dark-grown barley plants induces a massive insertion of the LHCP. The appearance of the protein is controlled by the cooperation of at least two distinct photoreceptors: protochlorophyllide and phytochrome. In dark-grown barley plants not only the LHCP but also its mRNA is not detectable. The light-dependent appearance of mRNA activity for the LHCP is under the control of phytochrome (Pf,). Even though the appearance of mRNA activity is induced via Pf, by a single red light pulse, the assembly of the complete LHCP takes place only under continuous illumination, which allows chlorophyll synthesis. The second protein analyzed so far is the NADPH-protochlorophyllide-oxidoreductase. This enzyme catalyzes the lightdependent reduction of protochlorophyllide to chlorophyllide and thus controls one of the first detectable light-dependent reactions during the greening period. It is generally assumed that this enzyme is responsible for the overall chlorophyll synthesis and accumulation during the greening period. In contrast to this hypothesis, we found a rapid decline of the enzyme during illumination. In addition to the decrease of the enzyme protein, the translatable mRNA coding for the enzyme also declines rapidly under the influence of light. Also this effect is mediated by phytochrome. Using cloned cDNA as hybridization probes we have demonstrated that the light-induced changes of the two translatable mRNAs for the NADPHprotochlorophyllide oxidoreductase and the LHCP are both paralleled by corresponding changes in the steady-state concentration of the mRNA sequences. Thus, it seems likely that one major point of control at which phytochrome regulates the development of chloroplasts is the expression of genes at the level of transcription.