ABSTRACT: Isotopic (18O and 15N in NO3 and 18O and 2H in water) monitoring and identification of the nitrate pollution sources in the groundwater from agricultural activities in Slovakia – was performed within the framework of the Nitrate directive (Council Directive 91/676/EEC) between the years 2016 – 2021 in Slovakia. 21 localities in agriculturally intensive areas were selected on the basis of nitrate concentration, geology, hydrogeology, and hypothesized pollution sources. This article presents and interprets data from two monitoring wells with particularly high nitrate concentrations and different temporal behaviour. 1) Livinské Opatovce, located in brackish-water sediments with alternated clay and sand. Elevated nitrate concentrations were identified in some years (up to 450 mg·L-1), whereas most of years showed concentrations below the limit. Seasonal changes in the isotopic composition suggest a relation to land use and agricultural practices, with depleted 15N-NO3 values associated to increasing nitrate concentrations. A organic origin of the nitrate contamination was identified, probably from organic fertilizers. 2) Tekovský Hrádok, located in fluvial gravels and sands. Nitrate concentrations were more stable, above the limit in all years, with a peak of 1700 mg·L-1 in 2011. Isotopic content of δ15NO3 = 5‰ suggests soil nitrogen, which is in contrast with high nitrate concentrations. Possible mixing of two sources of groundwater or a nitrate inflow from NH4 in organic fertilizers are hypothesized. In both cases, the isotopic composition suggests that the most probable source of nitrate pollution can be identified as organic, due to unsuitable agricultural practices such as overuse of manure and organic fertilizers.