Evidence is presented to support the conclusion that pore fluid pressures PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stress &sgr; lead to signficantly increased porosity and permeability, even at moderate to high metamorphic temperature. Permeabilities between 10-18 and 10-15 m2 are considered to be common, resulting in rapid fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high-pressure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control, is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.