Using a monoclonal antibody raised against the R1 subunit of the rat NMDA receptor, we mapped the distribution of NMDA receptors in the brains of three genera of electric fish. On Western blots, the antibody recognized a glycoprotein of approximately 105 kDa throughout the CNS. On tissue sections, it strongly labeled a number of neuronal somata and dendrites in the medulla, with weaker immunoreactivity in the forebrain and across much of the rest of the nervous system. At the ultrastructural level, reaction product was localized, though not exclusively, to the postsynaptic region of synapses. To study the role of NMDA receptors in a specific neural circuit, we focused on the medullary pacemaker nucleus. Neurons in this nucleus, which fire action potentials regularly and trigger each electric organ discharge (EOD), receive glutamatergic input from identified premotor areas. Activity in these areas can cause the pacemaker nucleus to produce outputs with distinct temporal dynamics, which are observed in the behaving animal as modulations of the EOD. The projection cells of the pacemaker nucleus, the relay cells, were heavily labeled with the anti-NMDA R1 antibody in all genera studied. These results are consistent with the previous finding that a particular EOD modulation mediated by the connection from one premotor area of the brain to the relay cells is blocked by application to the pacemaker nucleus of NMDA receptor blockers. Our results complement ongoing efforts to study this nucleus and provide additional evidence for the role of NMDA receptors in diverse neural circuits.