Strychnine is shown to abolish left-right alternation in fictive locomotion induced by sensory stimulation. Robust rhythmic activity, characterized by left-right coactivation at each segmental level, is seen in the presence of strychnine at all doses used (0.5-20 microM). The proportion of the cycle occupied by the ventral root bursts and the rostral-caudal coordination is similar to that seen in the absence of strychnine. Furthermore, the rhythm is abolished by cis-2,3-piperidine dicarboxylic acid (PDA), 2-amino-5-phosphonovalerate (APV), or the removal of Mg2+ from the perfusate, as in the absence of strychnine. Voltage clamp was applied to ventral horn neurons during stimulation in the presence of strychnine, with holding potentials negative to the plateau potential associated with a ventral root burst but positive to the potential in the interburst. Inward current was seen during the ventral root burst, but no outward current was seen at burst termination or during the interburst. The results indicate that in fictive locomotion induced by endogenous release of NMDA receptor agonists, left-right alternation is dependent on glycinergic transmission. Furthermore, evidence is provided that in the absence of glycinergic transmission, burst termination may depend on NMDA receptor-linked voltage-sensitive processes.