There is limited available data on natural rock fractures with low hydraulic transmissivity. In this study, we measured the transmissivity of a granite specimen with a 200×200 mm naturally-induced closed fracture under varying normal load. We repeated the measurements after opening the fracture to investigate how this affects its transmissivity. At each stage, we ensured laminar flows by setting Reynolds numbers << 1. Finally, we resorted to the parallel-plate theory to obtain the equivalent hydraulic aperture, that we compared to the mechanical aperture derived from the observed deformation.