Weakly electric fish generate electric organ discharges (EODs) that are species-specific and often sexually-dimorphic. The waveform or frequency of an EOD can be altered by treating a fish with sex steroid hormones. The EOD is controlled by a few discrete nuclei in the brainstem and spinal cord and a muscle-derived electric organ. The organizational simplicity and steroid-sensitivity of the electromotor system make it a premier system for investigating how sex steroids modulate behavior, neural circuitry, and ion channels. In addition, the diversity of EOD patterns in the many species of electric fish provide a wealth of material in which to examine the evolution of sexual dimorphisms in the nervous system.