We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2-m.y.-old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190¿10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350--370 m. indicating that ocean bottom water was flowing down the hole into the upper ~100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 6000--7000 1/h in late 1979 to about 1500 1/h 2 years later: altogether over 50¿104 kg of seawater has been drawn into the basement. We estimate a permeability of ≥6¿10 14m2 for the reservior in the upper ~100 of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a).