Electric fish evaluate the near environment by detecting changes in their self-generated electric organ discharge. To investigate impedance modulation of the self-generated electric field, this field was measured at the electrosensory fovea of Gymnotus carapo in the presence and absence of objects. Changes in local fields provoked by resistive objects were predicted by the change in total energy. Objects with capacitive impedance generated large variations in the relative importance of the different waveform components of the electric organ discharge. We tested the hypothesis that fish discriminate changes in waveform as well as increases in total energy using the novelty response, which is a behavioural response consisting of a transient acceleration of EOD frequency that can follow a change in object impedance. For resistive loads, the amplitude of novelty responses was well predicted by the increase in total energy. For complex loads, the amplitude of novelty responses was correlated not only with increases in total energy but also with waveform changes, consisting of reductions in the early slow negative wave and increases in the late sharp negative wave. The total energy and waveform effects appeared to be additive. These results indicate that G. carapo discriminates complex impedance based on an evaluation of different waveform parameters.