Fluid pressures have an important influence on rock strength in the brittle crust. This study investigates the development of fluid overpressures and the subsequent decrease in rock strength (differential stress at failure), induced by tectonic compression of a fluid-saturated poroelastic medium limited by a Mohr-Coulomb failure surface. It is shown that the pore pressure induced by compression of a poroelastic medium is controlled by the rate of loading relative to the rate at which the fluid pressure can dissipate by diffusion. Excess pore pressures may be generated by this loading mechanism if permeabilities are low, if the boundary deformation rate is fast, or if the distance between drained boundaries is large. The magnitude of the pore pressures developed, and the subsequent strength decrease, is controlled by Skempton's coefficient B, the undrained Poisson ratio &ngr;u, and the friction angle ϕ. Numerical results for realistic parameter values indicate that in upper crustal settings characterized by rapid stress increases (5¿10-10 MPa s-1), drainage length scales on the order of a few kilometers, and moderate to low permeabilities (10-18 to 10-16 m2), the strength may be reduced to ~60% of the strength of rock containing a hydrostatic fluid pressure and to 40% of the strength of dry rock. Under undrained conditions, fluid pressures are generated in proportion to the magnitude of the differential stress. Even so, fluid pressures generated by compressive (plane strain) deformation under undrained conditions are unlikely to attain the lithostatic pressure (i.e., pf/(&rgr;rgz)<1). Approximate analytic expressions are presented which enable a priori determination of compression-induced fluid pressures, and resulting rock strength modifications, in the brittle upper crust. ¿ 2001 American Geophysical Union