The influence of a fluid pressure load on the extensile fracturing of mortar and sandstone has been investigated. A fluid pressure in the (initiating) fracture stimulates both fracture initiation and propagation and may be as effective as a directly applied uniaxial tensile stress. The efficiency of the fracture pressure depends on the degree of saturation For the saturated materials, a pore pressure is generated under loading which counteracts the applied stress. The effective stresses governing the mechanical behaviour of the saturated materials then are lower, giving rise to an apparent stiffer behaviour, a lower effective fracture pressure and higher fracture initiation and propagation stresses. The magnitude of the pore pressure depends, among others, on the permeability of the materials. For impermeable materials, the pore pressure increases linearly with the hydrostatic stress, for permeable materials it is equal to the applied fluid pressure. With the increase in pore pressure, the effective stresses reduce and may even become zero when cohesion of the material is low.