Phytochrome, activated by continuous red light, increases the amount of total polyadenylated RNA during photomorphogenesis of mustard (Sinapis alba L.) cotyledons. In-vitro translation of total polyadenylated RNA in a reticulocyte translation system has shown that the activity of translatable fl-amylase mRNA is increased by phytochrome about threefold in the 3-d-old cotyledons, based on equal amounts of polyadenylated RNA, and about eightfold on a per-cotyledon basis. Cordycepin prevents the accumulation of translatable fl-amylase mRNA. It is concluded that the phytochrome-mediated control of/?-amylase synthesis is exerted on the level of mRNA synthesis. During seedling development in continuous red light, a phytochrome-dependent increase offl-amylase mRNA can be observed at least 6 h before the onset of fl-amylase synthesis. If, after a period of enzyme synthesis, phytochrome action is interrupted by long-wavelength far-red light followed by darkness,/?-amylase mRNA as well as/?-amylase synthesis remain at a high level for 8-10 h and then decline sharply. It is concluded that /?-amylase mRNA, having an apparent lifetime of the order of 8-i0 h, can be formed under the influence of phytochrome during early seedling development but it activates/?-amylase synthesis only after a lag-phase of about 8 h, when the cotyledons acquire competence to synthesize the enzyme. The consequences of these findings for the signal-transduction chain of phytochrome are discussed.