Chemical modification of purified nitrate reductase (NR) from sunflower leaves by white light-irradiated rose bengal was studied. NADH:NR activity was inhibited by light-activated rose bengal in both a concentration- and time-dependent manner. MV:NR activity was less sensitive to inhibition than NADH:NR activity, especially when the enzyme was preincubated with NADH. Preincubation of the enzyme with FAD protected inhibition of NADH:NR activity but not the MV:NR activity. These results suggest that sunflower NR contains sensitive histidine residue which interacts with reduced FAD during catalytic electron transfer. Most importantly, NADH-reduced NR was more sensitive to the irradiated dye, indicating that conformation of the oxidized and reduced enzyme forms were different.