The authors study the optical perturbations of the medium when sliding sparks (SS) are employed for preionization or optical pumping of the active medium in lasers. The experiments were performed on a Schlieren setup. A series of photographic frames are shown of the channel of an incomplete SS. It is shown that the SS are a source of perturbations of the gaseous medium in both the completed and incompleted stages of the discharge. The perturbations appear because of the nature of the formation of the SS and the form of the voltage applied to the discharge gap. The characteristic propagation velocities of these perturbatins range from 0.4 to 1 km/sec. This circumstance should be taken into account when employing SS as a source of preionization of laser media or as a source for optical pumping.