Fluid inclusion studies indicate that quartz veins represent pathways of rapid fluid movement in the Connemara Schists, where fluids have passed through contrasting lithologies without re-equilibrating. This suggests that fluid flow over distances of tens of metres must have occurred within a matter of hours. It is further suggested that since fluid generation is slow, fluid pressure and permeability may fluctuate during prograde metamorphism, with periodic episodes of hydraulic fracturing and fluid loss after slow increases in fluid pressure. Veining does not always occur during metamorphism and, where veins are absent, fluid loss may have been achieved through enhanced permeability or fine scale pervasive fracturing. Hydraulic fracturing is more likely if deviatoric stresses are large, and an association of veining with belts of more intense deformation is noted.