The search for a permanent storage facility for the geological disposal of high-level nuclear waste has motivated extensive research during the past several decades to characterize and predict fluid flow into and through unsaturated fractured rock. Tension infiltrometer experiments are extremely useful to investigate infiltration into fracture networks, but are difficult to perform using commercially available equipment developed mostly for soils. Our objective was to develop a tension infiltrometer suited for accurate measurements of infiltration into fractured rock at very low flow rates and for long equilibration times. We constructed several prototype instruments from porous stainless-steel membrane, stainless-steel casing, acrylic tubing, several temperature-compensated pressure transducers, solenoid valves, and a data logger for automated control and data acquisition. An automated refill system was also developed to facilitate long unattended equilibration periods typical in infiltration experiments on unsaturated fractured rock. Results show that the improved design reduces temperature effects on the infiltration rate, allows for much longer periods of unattended operation (auto-refill), and reduces evaporation from the infiltrometer. The estimated upper flow-rate limit of our new infiltrometer is about 1 mm d−1, based on the conductance of the porous steel membrane (11 mm d−1). We were able to make measurements of the fluid flux as low as 10 mm yr−1 at a pressure head of about −110 cm.