Intersegmental co-ordination in undulatory locomotion was investigated in the lamprey spinal cord in vitro. It is characterized by a constant phase lag between the activation of consecutive segments. By increasing the excitability (NMDA level) locally, a "travelling wave" could be induced to start in any segment and to be conducted in both rostral and caudal directions. This leading segment, the one with the highest excitability, determined the frequency of all other segments, and the ascending and descending couplings were functionally symmetric. The intersegmental phase lag can be explained by a simple neural mechanism in which a leading oscillator will entrain the adjacent "trailing" segments, which in turn entrain their neighbours and so forth.