Hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the Barbados Ridge accretionary complex. At low excess pore pressures [λ* = (Pp–Ph)/ (Pl–Ph) = 0.0 to 0.36, where Pp = pore pressure, Ph = hydrostatic pressure, and Pl = lithostatic pressure], estimated permeabilities were comparable to those of similar, unfractured sediment. These tests complement shipboard packer tests completed at higher fluid pressures (λ* = 0.5 to 1.0) during Ocean Drilling Program (ODP) Leg 156. Together, the test results suggest a 4- to 5-order-of-magnitude permeability increase as fluid pressure varied from hydrostatic (λ* = 0) to lithostatic (λ* = 1). However, unlike the results of the shipboard packer tests, the test results presented here exhibit no evidence of a relationship between permeability and pore pressure. The combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure.