Ground water in the Horná Nitra Basin and its close surroundings were dated by means of environmental isotopes. The thermal water at Bojnice is of Holocene origin. Ground water of the Mesozoic carbonate rocks underlying the Tertiary filling of the basin originated during the Pleistocene. Ground water of the Neogene sediments underlying the coal seams are unusually isolated of the present hydrological cycle, have a different residence time and start circulation only after the change of the hydrodynamic conditions by drilling or mining operations.