It is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain-size-sensitive deformation mechanisms resulting from the formation of fine-grained products of a metamorphic reaction. During prograde metamorphism, such effects are likely to be transient, because grain coarsening and textural equilibration are likely in response to rising temperature conditions. Thus deformation mechanisms are often difficult to infer from such naturally deformed rocks.In localized shear zones exhibiting retrogressive metamorphism, evidence of enhanced deformability by such mechanisms is most likely to be preserved, because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved.An experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure. A marked weakening (near-linear viscous rheology) at low strain rates was observed in association with the onset of dehydration to olivine. The enhancement of deformability is interpreted as due to the formation of thin, planar zones of ultrafine-grained but equiaxed (0.25 μm) olivine, which deform by diffusion-accommodated grain-boundary sliding. The experimental data therefore support the idea that a great deal of natural deformation during prograde metamorphism may occur in association with the transient existence of fine-grained reaction products, followed by grain growth and textural equilibration under essentially static conditions of relaxed stress.