The water distribution systems management often requires the definition of the relationship between leak outflow and flow conditions inside the pipe. To explore this relationship, several tests were carried out at the Water Engineering Laboratory of the University of Perugia (Italy). To analyze the leak outflow dependence on functioning conditions for different pipe materials, leaks with the same geometry were machined on steel and polyethylene pipes. The results of the test on the polyethylene pipe confirm the viscoelastic behavior of the leaks in these pipes, i.e. the leakage dependence on the functioning conditions time-history. On the contrary, the tests in the steel pipes of different thickness seem to confirm an elastic or elastoplastic behavior of the leaks. Since the considered test conditions are similar to those of functioning systems, it seems reasonable that the shown effects of the material properties on the leakage, especially for polyethylene pipes, can be relevant for water distribution systems management.