Agricultural  use  of land  on  hydrogenic  soils  is associated  with   lowering  the groundwater level, which intensifies the processes of organic matter  mineralisation  and,  in effect, releases   large  amounts  of mineral   nitrogen. The aim of the study was to determine the seasonal  variability of  the concentrations   of  nitrogen mineral  compounds (N-NO2, N-NO3  and N-NH4) in  groundwater   under extensively  managed  peat-muck  soils,  situated in  Wrocikowo  in  the  Olsztyn  Lakeland  and  in  the  Dymerskie  Meadows  in  the  Mr&gowo Lakeland. Peat-muck  soil,  found at  the  study  sites,  had  varied  physical  properties.  The highest  content  of mineral   particles (69.3%)  in the  muck  layer was  found in  soil marked  as MtI 120gy under  turf-covered  wasteland, and in  the peat layer (51.5%) - in  soil marked  as MtII 60gy under  extensively  managed  meadow. Groundwater  taken   from  piezometers installed  in  hydrogenic  soils  was  used as  the  study  material. Water  for  chemical determinations  was  taken  during four seasons:  spring  - in  May,  summer  - in  August, autumn  -  in November  and winter  - in  January. The  concentration   of mineral   forms  of nitrogen  in groundwater  of peat-muck  soil  was  found to depend on its  type,  the  depth where the  groundwater   was  found  and  the  type  of soil  use.  The  highest concentration of mineral  nitrogen was found in summer (1.62 mg dm-3 on average), and the lowest (1.11 mg  dm-3  on average) was  found  in  winter.  The concentration   of mineral   nitrogen  in groundwater   of extensively managed  peat-muck  soil  ranged  from  0.81  to 2.27  mg dm-3 and was  found  to be dependent on the  type  of soil  rather  than  its  use.  Lowering  the  level  of groundwater  in peat-muck  soil  increases   the  concentration  of mineral   forms  of nitrogen, especially  N-NH4. Ammonium  nitrogen  was  the  dominant  form  of nitrogen  in the groundwater   of peat-muck soil  (MtII 60gy and  MtI  120gy) of non-managed and extensively managed  meadows,  whereas  nitrate nitrogen  was  the  dominant form  of the  element  in  the MtIc   35gy  soil  under the  meadow and  the  MtII bb under  the  pasture.