Rel. contents of the N-15 and O-18 isotopes in nitrates contained in waters supplied the Solina reservoir by the San river, PL, were used for detn. of the origin of nitrates. They were mainly formed by nitrification in soil (47%), by conversion of sewage and natural fertilizers (30%) and in atmosphere (23%). The applicability of the isotope method for the identification of the nitrate origin was confirmed. Conclusions of pract. importance were presented.