A detailed geothermal survey has been made on the western flank of the Mid-Atlantic Ridge in the Brazil Basin where the crust is 20 m.y. old. The area is generally covered by a 150- to 200-m thick layer of foraminiferal ooze, but basement outcrops are numerous. Sedimentary temperature profiles indicate anomalously low temperatures at the base of the sedimentary layer, which results from cold bottom water penetrating laterally into the upper igneous crust and absorbing much of the geothermal heat. Net influx rates of 10-8 m s-1 are implied by a simple aquifer model. A negative pressure drop across the sedimentary layer is required by this interpretation. Refraction studies indicate low seismic velocities (3.3 to 4.5 km s-1) near the top of layer 2, and a large velocity gradient with depth. The velocity structure in the upper kilometer of the crust is related to the porosity, from which it is inferred that permeabilities there are high. Many of the 52 temperature profiles show a significant decrease in gradient with depth. The curvature can be attributed to one or a combination of the following causes: (1) upward advection of pore water at rates of 10-7 s-1, (2) bottom water temperature variations, (3) variation of conductivity with depth, and (4) experimental errors. Interpretation of curvature in terms of an advective model leads to results hard to reconcile with present knowledge of pressure variations and permeabilities of the oceanic crust.