Two-dimensional numerical calculations of hydrothermal circulation in permeable oceanic crust have been carried out. The effects of Rayleigh number, impermeable and permeable upper boundaries, and permeability variations with depth have been investigated. Flow and temperature fields as well as surface heat flux distributions are presented. Spatial distributions of surface heat flux are compared with observations at the Galapagos spreading center. The hydrothermal circulation alters the spatial distribution of surface heat flow but not its mean value. It is concluded that the oceanic crust has a permeability of about 4.5×10−12 cm2. With a permeable upper boundary the mass of seawater circulating through the crust equals the mass of the crust in about 2.1 m.y. A corresponding typical circulation time is estimated to be about 4 years.