In chloroplasts, carotenoids are essential pigments involved in photosynthesis. During-photomorphogenesis, a coordinated increase in the amounts of chlorophylls and carotenoids, in conjugation with other components, leads to the formation of a functional photosynthetic apparatus. To investigate the regulation of carotenoid biosynthesis during this process at the molecular level, GGPS, PSY and PDS cDNAs have been cloned from white mustard (Sinapis alba L). GGPS encodes a key enzyme in plastid isoprenoid metabolism, while the products of PSY and PDS catalyse the subsequent steps in carotenoid biosynthesis. Due to the low mRNA levels of the genes involved, the use of a RT-PCR protocol was necessary to measure gene expression during photomorphogenesis. With light, there is an up-regulation of PSY expression, the first gene within the carotenoid biosynthetic pathway, while PDS and GGPS expression levels remain constant. Treatment with different light qualities reveals a phytochrome-mediated regulation of PSY expression in developing white mustard seedlings. To obtain more detailed information on the light-regulation, Arabidopsis thaliana wild-type and phytochrome mutants were utilized. Continuous far-red and red light both increase the expression of PSY in wild-type seedlings, demonstrating that both light-labile and light-stable phytochromes are involved in PSY regulation. The response to far-red light is completely abolished in the phyA mutant, showing that PHYA mediates the increase in PSY transcript levels under these light conditions. In the phyB mutant, the red light response is normal, indicating that PSY expression is not controlled by PHYB but by other light-stable phytochromes. Measurement of chlorophylls and carotenoids under the same light regimes shows that the up-regulation of PSY expression does not necessarily result in an increase of the carotenoid content. Only those light conditions which allow chlorophyll biosynthesis lead to a significant increase of the carotenoid content. Therefore, it is proposed that up-regulation of PSY mRNA levels leads to an increased capacity for the formation of carotenoids. However, this only takes place under light conditions leading to protochlorophyllide photoconversion.