The pressure drop across a slugging fluidised bed increases with increasing gas velocity above the value representing the weight of suspended particles; this is in contrast to the situation for bubbling and homogeneously fluidised systems. The phenomenon appears to be largely due to the transformation, and eventual dissipation, of the potential energy developed by the rising solid slugs; it is fully quantifiable on this basis, and appears to be independent of the extent of particle-particle and particle-wall frictional interactions, both of which can play significant roles in other aspects of slugging behaviour. A mechanism is proposed for the energy transformation chain that gives rise to the observed pressure loss in the fluid.