Ammonium salts, glutamine, asparagine, and urea cause an immediate inactivation (switch-off) of light-dependent acetylene reduction in intact cells of the photosynthetic bacterium Rhodopseudomonas palustris. This effect is reversible showing the same kinetic pattern of inactivation and reactivation with all effector compounds. Its duration depends on the amount of effector added to the cells. Both nitrogenase components are found catalytically active in a cell-free preparation after enzyme switch-off in vivo. Involvement of the ammonia assimilating system in this regulatory mechanism is indicated by the following observations: ammonia uptake during the switch-off period, resumption of acetylene reduction after disappearance of ammonia from the outer medium, and persistence of enzyme switch-off with dihydrogen and thiosulfate as electron donors in the absence of an additional carbon source. Nitrogenase activity in crude extracts is non-linear with time and is stimulated by manganese ions. After resolution of nitrogenase into its MoFe--protein and Fe--protein these properties are lost, indicating the presence of an activating factor. Nitrogenase of R. palustris cross reacts reciprocally with the complementary proteins of Azotobacter vinelandii, but not with those of Clostridium pasteurianum.