ABSTRACT: Lazce karst spring in the Necpalská dolina valley of the Veľká Fatra Mts. with its average 558 L/s yield is the largest stable natural groundwater outlet in Slovakia. The spring groundwater originates in the Hrosková – Osičnô – Štefanová hydrogeological structure of Triassic carbonates in Veporicum tectonic unit outcropping between Necpalská and Ľubochnianska dolina valleys The size of this groundwater source accentuates the necessity for accurate determination of its recharge area, and although it is subject to legislative standards for water protection restricting economic activities no systematic discharge measurements have been made to ensure this spring’s proper water balance. To combat this inadequacy, we used the results of detailed hydrogeological mapping, current meter measurements, gauging results from three sources and isotope geology methods including d18O and d34S determination. According to recorded spring yields, the easternmost part of the Hrosková – Osičnô – Štefanová structure adjacent to the Ľubochnianka brook is considered balanced, and without hidden westward groundwater flow to the Lazce spring. Waters of the Beliansky potok brook from the top of the Belianska dolina valley are completely drained and we discovered no hidden surpluses from the Necpalský potok brook. However, in addition to the 34.5 km2 outcropping carbonates a further 9.0 km2 in three non-karstic rock areas must be added, because their water feeds the main carbonate structure.