Gymnotiform electric fish generate distinct communicatory signals by modulating the rate of their electric organ discharges (EODs). Each EOD is triggered by a command pulse from the medullary pacemaker nucleus (PN), which contains pacemaker cells and relay cells. The firing rate of this nucleus is modulated by inputs from the diencephalic prepacemaker nucleus (PPN). The NMDA receptor blocker APV and the kainate/quisqualate receptor blocker CNQX, administered to the PN, suppress different types of modulations, indicating that different classes of glutamate receptors mediate the generation of different modulations. A comparison of the 2 genera, Hypopomus and Eigenmannia, reveals that sustained modulations, such as smooth rises in the rate of pacemaker cell firing and the selective silencing of the relay cells (only observed in Hypopomus), are mediated by NMDA receptors, whereas the brief and rapid acceleration, called “chirp” or “decrement burst,” is mediated by kainate/quisqualate receptors. Application of the GABA blocker bicuculline reveals that the 2 genera differ in the mechanism by which they slow the firing rate of their pacemaker. Whereas Hypopomus uses GABAergic inhibition to slow down and ultimately silence its pacemaker cells, Eigenmannia reduces tonic, APV-sensitive excitation originating from its PPN and lacks GABAergic inhibition in the PN.