1. To elucidate the neural mechanisms responsible for coordinating undulatory locomotor movements, the intersegmental phase lag was analyzed from ventral roots along the spinal cord during fictive swimming. It was induced by bath application of N-methyl-D-aspartate (NMDA) in in vitro preparations of lamprey spinal cord, while the excitability of different segments were modified. The phase lag between consecutive segments during normal forward swimming is 1% of the cycle duration in a broad range of values. Rostral segments are activated before more caudal ones. 2. Under control conditions, whole preparations (12-24 segment long; n = 22) were perfused with NMDA solutions of the same concentration (100-150 microM). The intersegmental phase lag values varied in a continuous range with a single peak around a median value of forward +0.74% per segment (range: forward +2.23% to backward -0.97%). 3. To examine whether excitability differences along the spinal cord could modify the intersegmental phase lag, different levels of excitatory amino acids (NMDA) were applied to spinal cord preparations positioned in a partitioned chamber. Different portions of the cord could be perfused separately by NMDA solutions of different concentrations (50-150 microM). If rostral segments were perfused with the higher NMDA solution, the lag was inevitably in the forward direction. Conversely, if the caudal portion was perfused with the higher NMDA solution, caudally located ventral roots became activated before the rostral ventral roots in a caudorostral succession, thus reversing the direction of the fictive swimming wave to propagate as during backward swimming. If the middle portion was perfused by the highest NMDA solution, this portion instead became leading, and the activity propagated from this point in both the rostral and the caudal directions. The portion located in the pool with highest NMDA concentration always gave rise to a "leading" segment. 4. When a portion of the preparation was perfused with an NMDA solution of a high concentration (75-150 microM), the cycle duration was close to that recorded when the whole preparation was perfused with the same high NMDA solution. The ensemble cycle duration is, therefore, largely determined by the leading segment. 5. The phase lag changes were not restricted to the region around the barrier separating pools with different NMDA solutions.(ABSTRACT TRUNCATED AT 400 WORDS)