Evidence is presented to support the conclusion that porefluid pressures tresses PF during regional metamorphism aregenerally greater than or equal to the minimum principal compressive stress S3.The resultant very low effective stresses σ lead to significantly increased porosityand permeability, even at moderate to high metamorphic temperatures.Permeabilities between 10−18 and 10−15 m2 andare considered to be common, resulting in rapid fluid migration and thedominance of advective (infiltrative) over diffusive mass transport, even overrelatively small distances. In view of the importance of intergranular masstransport to rock deformation during metamorphism, a mobile, high-pressurefluid will have substantial rheological effects, especially in polyphase rocks.The fluid is capable of influencing the rate of dislocation creep in a numberof ways. More importantly, advective mass transport along fluid pressuregradients can give rise to a solution transfer deformation mechanism thatcompetes with conventional pressure solution. The rate of deformation byadvective mass transport could be controlled by a number of processes,including dissolution kinetics, advective transport rates, and the rate ofcrack growth. A specific deformation model, based on advective transport ratecontrol,is developed, which can produce strain at competitive rates but withstress and temperature dependences of unusual form.