In etiolated oat seedlings exposure to red light results in a decrease in the transcription of the phytochrome genes, the abundance of phytochrome mRNA, and the level of phytochrome protein. Phytochrome itself serves as the photoreceptor for the response of decreased mRNA and transcription levels. The decrease in phytochrome mRNA is sensitive to low levels of Pfr. Even green "safelight" is capable of inducing a decrease in phytochrome mRNA abundance. Barley phytochrome mRNA abundance is also dramatically down-regulated in response to red light but other plant species vary in their ability to decrease phytochrome mRNA abundance after red light treatment. Kinetic analysis and protein synthesis inhibitor data indicate that the abundance of phytochrome mRNA in oat seedlings may be regulated in part at the post-transcriptional level. Phytochrome mRNA may provide a useful model system for the investigation of posttranscriptional regulation of plant gene expression.