In the actively deforming materials of modern accretionary prisms there is a close interaction between permeability, mechanical state, and deformation structures. Core-scale structures from the Nankai prism, Japan, suggest that they have acted to localise fluid flow; dynamic permeability values (determined in the laboratory during progressive strain) vary with the strain state and, particularly, consolidation ratio. These interactions have been quantified further in experiments on sediments retrieved during recent ODP Leg 156 drilling of the Barbados prism and on laboratory-generated analogues. Dynamic permeability values vary in a complex manner with the amount of strain, and an important control is the degree of over- or under-consolidation. Moreover, because fluid pressure gradients give rise to variations in effective stress conditions across a sediment, the consolidation state differs locally. Material can be dilating in one part and showing an transient increase in permeability, while nearby areas are compacting and curbing flow. Detailed analysis of pore-volume changes during the experiments links intervals of dilation with the permeability/strain variations and accounts for the local but marked variations in the hydrogeology around the prism decollement. The results help explain evidence from the deformation structures in the decollement that indicate both fluid overpressuring and locally enhanced fluid flow, which implies transient pulsing of the pressurised fluids.