We present the first internally consistent calculation which leads to a narrow 'conduit' of rapid vertical advection and melting of mantle under a spreading center. In this model, mantle flow is driven by plate separation and compositional buoyancy. Melt segregation is described as flow through a permeable media. The major new feature is that the viscosity of the mantle is considered to be a strong function of the amount of partial melt present. Experiments show that the bulk viscosity of a partially molten rock is sharply reduced when the melt fraction exceeds a critical value. In the model, the viscosity is reduced as the critical melt fraction is approached. Whether or not a critical melt fraction can be reached under a spreading center depends on the mantle permeability for melt flow. The width of the upwelling area is controlled by the magnitude of the melt related viscosity reduction. Crust should be formed above the focused upwelling. Seismic observations show that the region of crustal accretion is only a few kilometers wide at fast spreading centers. With a viscosity reduction of three orders of magnitude the model predicts a zone of crustal accretion of this width. ¿ American Geophysical Union 1989