The purpose of the study was to determine seasonal changes of ammonia nitrogen, nitrate nitrogen, total nitrogen, phosphate and total phosphorus in the years 2003–2009 in the river Supraśl. For this purpose a monitoring network was created with selected three control points on the basis of which the assessment was made. Control points were located near Gródek, Nowodworce and Dzikie. The research results were averaged for each month. On this basis the assessment of seasonal changes of nitrogen and phosphorus forms in the river Supraśl has been carried out. Research and analysis showed that the concentration of nitrogen and phosphorus in waters of Supraśl were characterized by a clear seasonal variation, the highest concentration of nitrogen was observed in January and February and the lowest in June in each of the analyzed points, the course of changes in the concentration of nitric and total nitrogen in March were different from the general course of reigning changes in their concentrations in surface waters.