A mediated amperometric enzyme electrode sensitive to NADP + was developed by co-immobilization of ferrodoxin-NADP + reductase and viologen amino derivatives of different chain length (C 3 , C 6 and C 1 0 ) on the surface of a glassy carbon electrode. Glutaraldehyde cross-linking in the presence of bovine serum albumin was used for immobilization. Communication between the redox centre of the enzyme and the electrode through the immobilized mediator was proved, and a fast electrical response in the presence of NADP + was obtained. The best results were achieved with a medium-sized viologen. The sensitivity and linear response range depended on the polymer thickness and the immobilization procedure affected the stability of the modified electrode.