The molecular background of Hg 2+-induced inhibition of protochlorophyllide (Pchlide) photoreduction was investigated in homogenates of dark-grown wheat leaves. Our earlier work showed that 15 min incubation with 10-2 M Hg 2+ completely inhibits the activity of NADPH: Pchlide oxidoreductase ([Lenti et al., 2002]). Detailed analysis of spectra recorded at 10 K indicated the appearance of emission bands at 638 and 650 nm, which are characteristic for NADP+-Pchlide complexes. Fluorescence emission spectra recorded with different excitation wavelengths, fluorescence lifetime measurements and the analysis of acetone extractions revealed that Hg 2+ can also react directly with Pchlide, resulting in protopheophorbide formation. At 10-3 M Hg 2+, the phototransformation was complete but the blue shift of the chlorophyllide emission band speeded up remarkably. This indicates oxidation of the NADPH molecules that have a structural role in keeping together the etioplast inner membrane components. We suggest a complex model for the Hg 2+ effect: depending on concentration it can react with any components of the NADPH: Pchlide oxidoreductase macrodomains.