Investigation of nitrogen metabolism of phytoplankton, especially their nitrogen uptake, started in the eighties concerning to the eutrophication of  Lake Balaton. The experiments were carried out in two basins of the Lake and its two pollution control reservoirs (Marcali and Upper Kis-Balaton). Detailed description of these waters is given by Herodek and Vörös  in this volume. Due to the low ambient concentrations of ammonium, nitrate and urea the ¹⁵ N technique was the only useful method to determine the nitrogen uptake of algae. N₂-fixation was measured by acetylene-reduction and the primary production by ¹⁴ C method. The light dependence of nitrogen and carbon uptake was described with an exponential saturation equation and used for the calculation of surface related daily uptake. The results showed a high preference of algae for ammonium in all of the investigated waters. Its contribution to the daily nitrogen supply of phytoplankton varied between 50 - 90%. Despite its higher ambient concentrations nitrate uptake was generally less important with maximum of 40%. The interaction of ammonium  and  nitrate uptake demonstrated suppression of nitrate uptake at ammonium addition. After ammonium, urea was the other important nitrogen source of algae reaching up to 75% of daily nitrogen assimilation. N₂- fixation was significant only at the time of high primary production and great nitrogen fixing cyanobacterial contribution. Nitrogen uptake experiments and loading data of the Lake suggest that annual external nitrogen load is one tenth of that utilised by algae. The predominant process supplying nitrogen to the phytoplankton is the regeneration of nitrogen in the water column and in the sediment.