A two-dimensional cellular automaton model based on the elastic mechanics of tensile (mode I) microcracks was developed to investigate the evolution of rock strength and hydraulic connectivity during progressive dehydration. Fluid produced by dehydration is assumed to be accommodated by microcracks, which propagate through a simulated rock matrix owing to elevated fluid pressures. Crack propagation affects rheology by stress relaxation and interaction, and it affects hydrology by permitting fluid flow between neighboring cracks. Numerical simulations with undrained boundary conditions show that reactions releasing small quantities of fluid (<0.25 wt %) in a rock matrix with zero initial hydraulic connectivity induce large strength reductions (approximately 80--90%). Strength reduction occurs abruptly at the onset of dehydration and continues until approximately 10% reaction, when a low-strength plateau is reached. Subsequent reaction causes almost no further effect on rock strength until the percolation threshold is attained, at which point the strength drops to zero. Results with drained boundary conditions yield similar strength reductions before hydraulic connectivity of the crack network is achieved. Thereafter, fluid drainage allows partial strength recovery. The results indicate that the dominant rheological response induced by dehydration is caused by the generation of fluid overpressures and is unrelated to the establishment of hydraulic connectivity coinciding with the percolation threshold. Although rocks characterized by zero initial hydraulic connectivity retain additional strength relative to rocks with initial hydraulic connectivity, the magnitude of this additional strength is small (<20% original strength). ¿ 1999 American Geophysical Union