Description: Previous models and calculations of the global mass balance of Sr in the oceans have shown that the input of unradiogenic basaltic Sr from on-axis midocean ridge hydrothermal systems is much less than needed to balance the input of radiogenic Sr delivered to the oceans by rivers. The implication is that either the oceans are far from steady state with respect to Sr isotope balance (and that the 87Sr/86Sr ratio of seawater is increasing at unprecedented rates) or that there is a significant missing source of basaltic Sr. It has long been recognized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr and other elements in the oceans, but nearly all previous work has concluded that the 87Sr/86Sr ratio of pore fluids in ridge-flank hydrothermal areas is virtually indistinguishable from the seawater ratio or is dominated by authigenic carbonates. In contrast, we report here the 87Sr/86Sr ratios of warm springs, sediment pore fluids, and basement reservoir fluid with a clear basaltic signature from the eastern flank of the Juan de Fuca ridge (JFR). Fluids venting from Ocean Drilling Program Hole 1026B on the Juan de Fuca east flank have relatively stable Sr isotope and major element composition for the 3 yr following drilling. These results and similar results recently reported by Elderfield et al. (1999) indicate that low-temperature ridge-flank hydrothermal circulation has an important effect on the Sr isotope balance in the oceans. If published values for the other major sources of Sr input to the oceans (rivers and axial hydrothermal flux) are accurate, then the rate of increase of the 87Sr/86Sr ratio in seawater (~0.000054 per million years) can be accommodated if ridge flanks on a global scale deliver fluids to the ocean with delta (87Sr/86Sr)/heat ratios one third to one half of the ratio found in warm JFR basement fluids. Based on published Sr and O isotope signatures of calcite veins in the uppermost basaltic ocean crust, the average delta (87Sr/86Sr)/heat ratio of low-temperature fluids is in the range required to balance the oceanic Sr isotope budget. Although the 87Sr/86Sr ratios of the JFR flank fluids in this study overlap with fluid properties inferred from some calcite veins in the upper oceanic crust, the magnitudes of the delta (87Sr/86Sr)/heat ratios of nearly all of the JFR flank fluids are too large to be representative of the average global flank fluid flux; the same has been argued on the basis of the extremely high implied Mg flux.