The greening of 5-d-old etiolated Sorghum seedlings was used to investigate regulatory mechanisms of plastid gene expression. Sorghum was selected for this study because leaves of etiolated seedlings do not expand in the dark and are kept within the coleoptile, thus leaf development is inhibited during etiolation. Only traces of thylakoid-membrane polypeptides and plastid transcripts could be detected in dark-grown seedlings, the accumulation of both protein and RNA was found to be strictly dependent upon light. Run-on transcription analysis demonstrated that the rise in plastid RNA levels was due to a light-induced increase in plastid transcriptional activity and not to altered RNA stability. Using an antiserum to the β subunit of plastid RNA polymerase, it could be demonstrated that the increase in plastid transcriptional activity was paralleled by rising levels of plastid RNA polymerase. It has to be concluded, therefore, that in greening Sorghum seedlings the expression of plastid genes is photoregulated at the transcriptional level.