These experiments were carried out to investigate whether accumulation of carbohydrate leads to decreased expression of genes involved in photosynthesis. Addition of glucose to autotrophic cell suspension cultures of Chenopodium led to a large and reversible decrease of the steady state transcript levels of rbcS, cab and atp-& within 5 h, but did not decrease 18S rRNA or transcript for two glycolytic enzymes. Run-on transcription in isolated nuclei showed that transcription rate had been decreased. [35S]Methionine feeding showed that de novo synthesis of Rubisco was inhibited. Decreased rbcS transcript was also found after feeding glucose to detached leaves, and in transgenic plants expressing invertase in the apoplast to inhibit phloem transport, and in leaves on intact tobacco and potato plants which were cold-girdled to decrease export. The decrease of rbcS transcript level occurred within 12 h of coldgirdling. Comparison of carbohydrate content and rbcS transcript level indicated that carbohydrate content per se is not the direct signal for regulation of gene expression. Feeding of transported analogues indicates that metabolism rather than transport of the sugars is required. Over-expression of rbcS was found in low CO2, again indicating metabolic control of expression. It is proposed that photosynthetic gene expression is inhibited by metabolic factors related to high carbohydrate content, and that this represents a basic mechanism for the ‘sink regulation’ of photo-synthesis.