A new apparatus is designed to measure the permeability of porous asphalt road mixes under laboratory conditions. This apparatus works on the falling head principle, from which the falling head with time through a porous specimen is measured with a pracise pressure transducer at very close time intervals (10 readings/s). Using the falling head data, she specific discharge through the specimen can be computed and the penneability of the material obtained from the velocity versus hydraulic gradient relationship. Tests on unbound specimens made with glass spheres showed excellent agreement with constant head test results. The influence of specimen diameter and thickness in relation to particle size is examined, and recommendation for appropriate specimen size proposed. Tests with three types of common porous asphalt mixes were conducted and it is demonstrated that specification of void content alone is not adequate for describing the drainage property of a porous mix.