Experiments described in the literature have demonstrated that additions of N, Fe, and occasionally P, influence planktonic cyanobacterial nitrogen fixation in lakes and estuaries throughout the world. Increase in abundance of cyanobacteria which can fix nitrogen did not necessarily indicate that N2fixation had occurred. In natural plankton assemblages, N2fixation was normally stimulated by low total inorganic nitrogen (TIN) and depressed by additions of TIN. Nitrogenase was often stimulated by addition of Fe but soluble reactive phosphate (SRP) alone only stimulated nitrogenase activity occasionally. Luxury consumption and storage of P, but not N, explains the lack of P stimulation in nature. Nitrogenase activity was usually repressed at TIN oncentrations of >50–100 μg l‐1. Additions of N + P had variable effects which may depend on the balance between nitrogenase inhibition by N and general growth stimulation by N + P (which reduces ambient N).