The mesotrophic Lake Piaseczno and the eutrophic Lake Głębokie (in the Łęczna-Włodawa Lake District) were the subject of a five-year study. The activity of the process relating to nitrogen cycle (protease, ammonification) and phosphorus cycle (alkaline phosphatases total, algal, bacterial and free) in the water and the bottom sediment in various zones of these reservoirs were investigated. Both in the water and bottom sediment or Lake Głębokie a higher biochemical activity than that in Lake Piaseczno was observed. Differences in activity appeared stronger in the case of phosphatases. It would suggest on the usefulness of these tests for the evaluation of the lakes trophic level.