The parameters determining the annual balances of nitrogen (N) and the concentrations of total N in lakes were analyzed. This was carried out by the use of multiple regression analyses on data from a number of northern temperate lakes with a hydraulic detention time of 0.25 yr or more. The result indicated that the removal of N in a lake, which in most cases is dominated by denitrification, seems to be controlled by organic matter. If no major N fixation is taking place, the concentration of total N in the epilimnion during summer is proportional to the load of N and inversely proportional to the concentration of total P or chlorophylla. For some lakes N fixation is taking place. The increase in total N due to N fixation was found to be proportional to the concentration of total P reduced by a fraction of the estimated N concentration without any fixation. For most lakes, the effect of N fixation is to balance the N content with the P content, which means that the algal biomass is limited by the P content. A ratio of the total concentrations N/P less than approximately 10 indicates a rate of denitrification exceeding the capacity of N fixation. N is not balanced with P, and the algae is controlled by N.