Mid-ocean ridge hydrothermal vent fields are characterized by maximum sustained venting temperatures of 320¿--380 ¿C, irrespective of the spreading rate and the inferred depth of circulation. Metamorphic assemblages, fluid inclusions, oxygen isotope data, and the salinity of vent fluids have all been used to infer maximum circulation temperatures of up to 500¿--700 ¿C. In this paper I investigate the pattern of circulation and the relationship between venting and basal temperatures using simple models of steady open-top porous convection in a rectangular medium heated from below. Solutions obtained with variable fluid properties which approximate seawater include much more recirculation than solutions obtained with uniform fluid properties, and the temperatures of upwelling fluids are greater. For a uniform permeability, models obtained with seawater properties yield a ratio of the venting temperature to the bottom temperature in the range 0.5--0.65, values which are reasonably consistent with the observations. However, a surficial layer of high permeability significantly reduces vent temperatures. Since extrusive basalts are almost certainly very permeable, this result seems incompatible with the existence of black smoker vents. It has been suggested previously that the precipitation of hydrothermal minerals where hot and cold fluids meet will lead to the formation of an impermeable shell around upflow zones. Such a shell may provide a means to tap hot fluids from depth. ¿ 1998 American Geophysical Union