Ferredoxin:NADP(+) oxidoreductase (FNR) was treated with cadmium and after that its diaphorase reaction in the presence of dibromothymoquinone (DBMIB) or ferricyanide (FeCy, K(3)Fe(CN)(6)) was examined. CdSO(4) (5 mM) caused 50% inhibition after half hour incubation. At least two components were distinguishable in the time-course inhibition, suggesting that more than one amino acid residues were engaged in reaction with the metal ion. The Lineweaver-Burk plots indicate that Cd(2+) is an uncompetitive inhibitor for DBMIB reduction but exerts non-competitive inhibition for the NADPH oxidation. The FeCy reduction did not follow Michaelis-Menten kinetics. Zn(2+) diminished inhibitory effect of Cd(2+) on the DBMIB reduction but enhanced inhibition of the FeCy reduction. Incubation with additional chelator (beta-mercaptoethanol, or histidine) abolished inhibitory effect of Cd(2+) on the FeCy reduction but not on the DBMIB reduction. The mode of Cd(2+) action on the diaphorase activity of FNR in the presence of DBMIB or FeCy is briefly discussed with the special reference to the implication of two distinct sites at the FNR molecule, which might be involved in the reduction of various non-physiological substrates.