Results of numerical simulations of three dimensional hydrothermal convection in a layered medium representative of mid-ocean ridge environments are presented. Regional-scale three dimensional flow patterns naturally arise from reasonable assumptions about permeability distributions and magnitude of base heating. These flow patterns are characterized by broad downwelling but restricted regions of upwelling. Magnetic intrusions along the ridge axis induced vigorous convection restricted to the vicinity of the ridge-axis, whose character changes during the intrusion cooling phase. ¿ American Geophysical Union 1991