Hydrothermal vents along unsedimented portions of mid-ocean ridges are fed by flow confined to a 1--3 km high percolating system of fissures subparallel to the spreading center. We have solved the coupled thermal and flow equations in such a system. We assume that hydrothermal circulation occurs in a vertical porous slot 1.5 km high, 2.3 km long, and at most 100 m wide. In this model we take into account the heat transported by the hydrothermal flow inside the porous slot as well as the heat conducted through the surrounding impermeable crustal layer. The fluid is free to enter or leave the top of the slot. The fluid enters the slot at a temperature of 2 ¿C, while the bottom of the porous slot is held at a constant temperature of 420 ¿C. We first consider a vertical slot with a horizontal base. Our calculations show the development of tall and narrow unsteady convective cells. Because of the nonlinear relationship of the viscosity and density of seawater with temperature, the upper thermal boundary layer is much thicker than the lower one. The fluid advected in the hot plumes tends to remain confined inside the slot because the upper boundary is thick and viscous. In high Rayleigh number experiments, some very active hot plumes are able to thin and pierce the upper cold boundary layer, and vent hot fluids at the seafloor. The maximum temperature at the exit of these plumes reaches a value at most equal to 270 ¿C. The presence of an axial magma chamber along most of the length of the East Pacific Rise may justify the use of a horizontal base to model hydrothermal systems along fast spreading ridges. However, where the hydrothermal systems develop above a short-lived magmatic intrusion a few kilometers long, the lower interface of the percolating system of fissures is inclined. In that case our calculations show that the convective flow tends to be unicellular and steady state with a wide region of downwelling and a narrow upwelling zone. The temperature of the fluid exiting the slot at the top reaches a maximum value of 360 ¿C. These results lead us to suggest that hydrothermal plumes generated inside a fracture with a horizontal base give rise to vents which wander along the fracture plane with a maximum temperature of about 270 ¿C. However, when the base of the fracture plane is inclined, a large hydrothermal steady plume is generated which can give rise, at its exit, to a black smoker with fluid temperatures in excess of 350 ¿C. ¿ 1999 American Geophysical Union