A new experimental technique for the investigation of drilling-fluid induced borehole instability in shales is introduced. Pressure transmission transients are used to study pressure penetration in shales, the latter being a main shale-destabilising mechanism. The pressure transmission method provides a powerful tool for the fundamental study of shale-fluid interactions and for the development of improved water-based mud formulations to promote borehole stabilisation in shales. The capabilities of the method are illustrated by permeability measurements made in low-permeability shales. In addition, a wide range of conventional and new mud systems is screened and the shale-stabilising performance of these systems is assessed. It is argued that the strategy for promoting borehole stability in shales through improved mud formulation should be based on maintaining pressure isolation between wellbore and formation.