The electrical properties of r.f. planar magnetron sputtered ZnO films are studied by means of current-voltage, capacitance-voltage and Van der Pauw measurements. These films are applied as piezoelectric transducers in micromechanical sensors and actuators. Their piezoelectric behaviour strongly depends on the electric properties. A conduction model for the polycrystalline ZnO layers is presented. This model gives a good description of the electrical behaviour, and is useful in understanding the piezoelectric properties of the films studied.