Abstract 1. 1. Recordings were made from primary afferent and secondary neurons in the electrosensory system of the thornback ray. 2. 2. The frequency response characteristics of these neurons were determined during sinusoidal modulation of uniform external fields. 3. 3. The form or the filter curves was similar in all primary and secondary cells, with a maximum amplitude of response occurring at a modulation frequency of 4 Hz. 4. 4. Secondary neurons showed an increased gain at low stimulus amplitude, and saturated at a much lower level than primary afferents.