Changes in 87Sr/86Sr ratios of seawater may reflect varying rates of chemical weathering or variations in the isotopic compositions of continental rocks undergoing weathering. The unusually high dissolved Sr concentrations and 87Sr/86Sr ratios that characterize the Ganges and Brahmaputra-Tsangpo river systems are a consequence of weathering Early Proterozoic age metasedimentary rocks and anatectic Miocene granites exposed on the southern slopes of the Himalayas. On average, the metasedimentary rocks are characterized by abundances of 87Sr about twice those of the derived granites. The rapid rise in 87Sr/86Sr ratios of seawater during the early Miocene was virtually coincident with metamorphism and anatexis of Himalayan metasedimentary rocks. Model calculations indicate that 50%–75% of the overall change in 87Sr/86Sr ratios for seawater between 20 and 15 Ma could have resulted from exposure of Himalayan metasedimentary rocks to chemical weathering. Rapid, tectonically controlled exhumation of the metasedimentary rocks is required to expose these lithologies by 20–18 Ma, to generate the increase in the 87Sr flux of the trans-Himalayan rivers during the early Miocene.