During late Palaeozoic (Hercynian) low-pressure regional metamorphism in the Pyrenees, exceptionally high thermal gradients existed within the upper crust, and temperatures as high as 700 degrees C were attained at depths as shallow as 10 km, resulting in large-scale crustal anatexis. Stable isotope studies indicate that the crust was flushed by circulating ground waters to depths of 12 km, but the amount of fluid involved below 8 km was probably not much greater than 50% of the rock mass, and this fluid apparently did not penetrate the pre-Palaeozoic basement below 12 km. There is no evidence for continental collision in the region at that time, and these data, together with other geological and geophysical constraints, suggest that the most plausible tectonic setting for the metamorphism is a zone of continental rifting, possibly associated with strike-slip movement. Thermal modelling suggests that a transient, high-temperature heat source in the lower crust is required to account for the observed metamorphic P--T arrays. Among a range of possible solutions, a basaltic sill, 6-8 km thick and emplaced at 14 km could generate a maximum temperature array similar to those observed in the Pyrenees.