Rock masses containing a large number of geological discontinuities (called joints) are treated as homogeneous, anisotropic porous media. A permeability tensor, hydraulically equivalent to a flow network formed by joints, is formulated in terms of two tensors and a nondimensional scalar, both depending only on the geometrical aspects of joints such as the spacing, size, orientation, and aperture. Two-dimensional numerical analyses on seepage flow through the networks support the validity of the present theory. The information about the geometry of joints, which is obtained from the ventilation drift at Stripa mine, Sweden, is interpreted to give the corresponding permeability tensor. The permeability tensor is compared with the result of the large-scale hydraulic conductivity test, with the conclusion that it provides a reasonable prediction for the hydraulic properties of Stripa jointed granite. ¿ American Geophysical Union