ABSTRACT: Results of a study concerning the variability of nitrate and ammonia nitrogen concentrations in groundwater of peatlands as the effect of different dewatering degree and management systems are presented. The investigation was conducted in three different sites on the area of peatlands of the Biebrza River Valley in south-eastern Poland. All the sites were divided into zones of advancing bush and tree succession or ground cover. During field exploration, the quality of ground, surface and drinking water were monitored and changes of groundwater levels were recorded. The results of the investigation indicate a variation of nitrate nitrogen (N-NO3) concentration in groundwater between the sites, a relationship between ground cover and N-NO3 concentration and excessive concentrations of ammonium nitrogen in ground and surface water.