The isolated spinal cord of the neonatal mouse spontaneously generates two different motor patterns of continuous rhythmic bursting: one in lumbar ventral roots in earliest postnatal preparations (P0-2) and another at the sacral level at later postnatal times (P3-5). Lumbar rhythmic motor discharges clearly alternate on contralateral roots and are in a frequency range (approximately 1 Hz) usually described for locomotor-like activity, while sacral motor sequences include mixed synchrony and irregular bilateral alternation that differ from typical locomotor-like activity. A significant decrease in the frequency and increase in the duration of spontaneous rhythmic bursts occur between lumbar and sacral motor patterns. In quiescent preparations from both postnatal periods, perfusion with Mg(2+)-free medium systematically induces a rhythmic activity at both lumbar and sacral level. Temporal characteristics of motor patterns under Mg(2+)-free medium are similar to spontaneous rhythms. Activating NMDA receptor channels by diminishing their Mg2+ block appears to be an efficient way of decreasing the threshold for genesis of the spinal rhythm in mouse. Bath application of NMDA and non-NMDA receptor antagonists blocks Mg(2+)-free-induced rhythmic activities in an irreversible or reversible manner, respectively. The effects of Mg(2+)-free medium and of glutamate antagonists provide evidence for the excitatory amino acid (EAA) dependence of both rhythmic motor patterns. Finally, the possibility that the recording of two different motor patterns may reflect a rostrocaudal developmental process is discussed.