The aim of the studies was to determine the content of mineral nitrogen in the ground water. The studies were conducted during the growing season (June-October) of 2001 and at extra vegetative time (November-February) in 2001/2002. Ground water samples for measurement of the groundwater contents of NO3- and NH4+ were collected from the observation wells located on permanent meadow and the area of lysimeter station with organic soil, on the pasture established on black earth soil and pasture on peat-muck soil. Both soils are characterized by low availability of nitrogen. The level of ground water was differentiated depending on the type of soil and time of measurement. Additionally, water from the Piwonia river was taken for study. The content of nitrate and ammonia ions in the ground water was determined by LF-204 photometer, using the enclosed reagent set. The manner of grassland use, soil type and time of study significantly affected the content of the above ions in ground water and conditioned the rate of their displacement through the soils. Differences in the content of mineral nitrogen in ground water from meadow and pasture on organic soil were not important. The amount of nitrogen in ground water from mineral soil was higher than in water from peat-muck soil. Mean NH4+ concentration ranged from 0.16 to
1.02 mg l-1 and showed an increasing tendency during the summer, contrary to the content of nitrates (0.45–6.25 mg l-1), which was much higher in the ground water samples collected in autumn and winter. Increasing concentrations of nitrates in the ground water was confirmed after intensive rainfalls; therefore the quantity and precipitation distribution seems to be important for water pollution.