Two in situ bulk permeability tests were conducted in the metamorphic basement rocks in the Cajon Pass research well. In one interval, from 1829 m to 1905 m, we measured an effective permeability of 0.5 × 10−18 m² (0.5 × 10−6 darcy). Over an interval from 1829 m to 2115 m, we measured an average permeability of 1.67 × 10−18 m² (1.67 × 10−6 darcy). In addition, fluid pressures 5% over hydrostatic were determined through analysis of pressure build-up in the larger test interval. The in situ permeability measured is one to three orders of magnitude greater than the permeability measured in core samples; a difference we attribute to the presence of fluid conducting fractures. A large fracture at 2076 m with an aperture of 26.6 cm appears to be a zone of localized high permeability. The permeability of this fracture is as high as 1.8 × 10−16 m² (0.18 × 10−3 darcy). In light of the low permeability and fluid pressure measured, the transport of significant amounts of frictionally generated heat by hydraulic conduction is extremely unlikely.