HY5 and HYH are bZIP transcription factors well known to be involved in photomorphogenesis and light signalling. Loss-of-function mutants of HY5 and HYH revealed that these genes are essential for induction of a key enzyme in nitrogen assimilation, nitrate reductase (EC 1.7.1.1). In Arabidopsis thaliana seedlings nitrate reductase was expressed under low irradiance far-red or red light at the same level as under higher irradiance, photosynthetic active, white light. However, high NR expression at low light levels occurred only in the presence of sucrose in the growth medium. Sucrose did not promote expression in darkness. Whereas HY5 was necessary for high nitrate reductase expression in far-red light, HYH was important in red light. COP1 is known to promote degradation of HY5 and HYH, and in the cop1 mutant, nitrate reductase activity was relatively high also in darkness. PhyA and PhyB mutants were tested, and confirmed the phytochrome dependency for far-red and red light induction of nitrate reductase in seedlings. In rosette leaves of 3-week-old green plants the daily increase in nitrate reductase expression in response to light-on was abolished in the hyh and hy5 hyh double mutant. The hy5 hyh double mutant had lower nitrate reductase activity than any of the single mutants in photosynthetic active light in both seedlings and rosette leaves.