Experimental observations are reported of high-frequency currents associated with thin photoconducting cadmium sulphide platelets having flat and parallel faces. They appear under conditions when `round-trip gain' due to acousto-electric amplification is achieved. They are correlated with departures from Ohm's law. The results are confined to conditions under which a linear theory is believed applicable. The linear theory is outlined and comparison is made with a variety of experimental observations. Semiquantitative agreement is obtained.