For regional and local isotopic investigations, it is important to compare isotopic data of surface water and groundwater with the local meteoric relationship for 18O and d2H in precipitations. In this paper, the Mashhad meteoric water line (MMWL) is defined for the first time, based on samples of precipitation collected from several rain stations located within and around of Mashhad city - NE of Iran. Both the slope and d2H intercept for MMWL (d2H = 7.17 d18O + 11.22) are deviated from the global meteoric water line - GMWL (d2H = 8.13 d18O + 10.8). The huge variation in isotopic compositions of the Mashhad rain (from -198.2 to +38.1‰ and from -26.1 to +3.7‰ for d2H and d18O, respectively) is because of strong seasonal changes in precipitations in this area. The 2H and 18O isotopes of local water resources show evaporative enrichment in comparison with local meteoric water.

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