The photoreduction, without reductant dithionite, of N2 to NH3 or acetylene to ethylene catalysed by nitrogenase in the presence of Mg2+. ATP, eosin and NADH in the light has been established. There is an optimum NADH concentration for each particular eosin concentration. When the ratio of the iron protein component of nitrogenase from Azotobacter vinelandii (Av2)/the molybdenum-iron protein component of nitrogenase from A. vinelandii (Av1) is equal to 3 for 4 x 10(-5) M eosin the optimum NADH concentration is 5 x 10(-4) M. The rate of photoreduction (per one electron) of acetylene or N2 under identical conditions was shown to be similar. The photoreductant-dependent ATPase activity, in the presence of a given photochemical system in the light, was revealed. Eosin is shown to be the inhibitor of the coupling site. Concentrations of 8 x 10(-6) -1 x 10(-4) M eosin do not inhibit the ATPase activity. The inhibition of substrate-reduction activity depends on the ratio of the nitrogenase components. Under conditions where the Av2/Av1 ratio is equal to 1 the rate of photochemical reduction is higher than in the presence of dithionite: the total electron flux through nitrogenase being increased 2.2-fold. We suggest that in this case the nitrogenase complex (1:1) works without dissociation.