There are fundamental differences between low- and high-porosity rocks in the evolution of permeability in the brittle faulting regime. In a rock with porosity less than ~5%, dilatancy and permeability enhancement were observed prior to the peak stress. In porous rocks, the behavior is more complex. Before the onset of dilatancy C', both permeability and porosity decrease with increasing effective mean stress. Beyond C', permeability may decrease while the pore space dilates. After the peak stress has been attained, the development of a relatively impermeable shear band caused an accelerated decrease of permeability. A network model (with pore space statistics constrained by microstructural data) was developed to simulate the interplay of pre-existing tubular pores and stress-induced cracks in influencing the permeability evolution. The simulations show that while the accumulation of damage in form of microcracks may dilate the pore space, it causes the flow path to be more tortuous and as a result, the permeability may actually decrease. ¿ American Geophysical Union 1996