Hydrothermal fluid circulation within the sea floor profoundly influences the physical, chemical and biological state of the crust and the oceans. Circulation within ridge flanks (in crust more than 1 Myr old) results in greater heat loss(1-3) and fluid flux(4) than that at ridge crests and persists for millions of years, thereby altering the composition of the crust and overlying ocean(5,6). Fluid flow in oceanic crust is, however, limited by the extent and nature of the rock's permeability(7). Here we demonstrate that the global data set of borehole permeability measurements in uppermost oceanic crust(7-9) defines a trend with age that is consistent with changes in seismic velocity(10,11). This trend-which indicates that fluid flow should be greatly reduced in crust older than a few million years-would appear to be inconsistent with heat-flow observations, which on average indicate significant advective heat loss in crust up to 65 Myr old(3). But our calculations, based on a lateral flow model, suggest that regional-scale permeabilities are much higher than have been measured in boreholes. These results can be reconciled if most of the fluid flow in the upper crust is channelized through a small volume of rock, influencing the geometry of convection and the nature of fluid-rock interaction.