Brittle experimental deformation on dolomite rocks shows for the first time the difference in growth of fracture networks by ordinary percolation and invasion percolation processes. Stress-driven fracture growth, in the absence of fluid pressure, is an ordinary percolation process characterized by distributed nucleation and growth of microfractures, which coalesce with increasing strain to form a connected fracture network. Fluid pressure–driven fracture growth is more akin to an invasion percolation process characterized by preferential fracture growth occurring initially at the high fluid pressure part of the rock. With progressive deformation, the network propagates rapidly through the sample and away from the high fluid pressure reservoir. X-ray microtomography analysis suggests that the fracture network in three dimensions (3-D) is probably a fully connected network at peak stress conditions, whereas conventional 2-D analysis suggests that connectivity only occurs at shear failure. The development of 3-D fracture connectivity prior to shear failure has important implications for fluid flow and fluid pressure changes immediately prior to rupture nucleation in active fault zones, for fluid migration in ore-producing hydrothermal systems, and for reservoir integrity in hydrocarbon systems.