ABSTRACT: The Demänovská dolina Valley karst area belongs to the most important karst area in the Western Carpathians Mts. It is formed mostly by Middle Triassic Gutenstein limestones in the Krížna Nappe Veporic unit, where the hydrogeological settings, regime and groundwater circulation are influenced by the area’s geological-tectonic structure and degree of karstification. We registered extremely close relationships between groundwater and waters on the surface and in the karstic underground hydrological system. The vertical, subvertical and horizontal water circulation in the karstic underground hydrological system is extremely complex, but it has a typical underground stream network. The Demänovka Stream is the most important allochthonous underground stream, and while siphonal circulation is typical in its shallow phreatic (epiphreatic) underwater areas, groundwater circulation in the deeper phreatic zone remains unknown.