Primary transcripts in higher plant cell organelles undergo a series of essential RNA processing steps, including 5'- and 3'-terminal maturation, intron splicing and RNA editing. Most of the protein factors involved in these RNA processing mechanisms are still unknown. Also, little is known about the environmental and developmental factors regulating RNA processing events in plastids. Here we have tested the influence of the environmental factor temperature on RNA processing in the ndhB mRNA from tobacco plastids. We find that, whereas temperature increase to 37 degrees C fully inhibits RNA editing at only one out of nine sites in the tobacco ndhB transcript, further temperature increase to 42 degrees C selectively blocks editing at two additional sites. As these temperature effects are strictly site-specific, our findings provide evidence for the existence of site-specific editing factors which exhibit differential temperature sensitivity. Furthermore, splicing of the group II intron present in the ndhB pre-mRNA is largely blocked at 42 degrees C, suggesting that chloroplast RNA processing steps are more sensitive to high temperature than chloroplast transcription. Our findings may also suggest that impaired plastid RNA processing contributes to the loss of chloroplast function upon plant growth at high temperature.