1. Whole cell current-clamp recordings were obtained from tetrodotoxin (TTX)-isolated motoneurons in the in vitro neonatal rat spinal cord to examine the effects of N-methyl-D-aspartate (NMDA) receptor activation on membrane voltage. 2. NMDA induced rhythmic membrane voltage oscillations, and injection of current ramps revealed the presence of bistable membrane properties, the base and peak of which corresponded to the base and peak values of the voltage oscillations. 3. Nonlinear motoneuron membrane properties induced by NMDA receptor activation may be well suited to reinforce rhythmic patterns of motor output during certain behaviors such as locomotion.