1. The intersegmental coordination during undulatory locomotion in lamprey is characterized by a constant phase lag between consecutive segments, that is, the ratio between the intersegmental time lag and the cycle duration remains constant. It is shown that the spinal 5-HT (serotonin) system can, in a graded fashion, control the phase lag value from a rostrocaudal to a caudorostral lag corresponding to a reversed direction of swimming. These effects can be explained by a 5-HT-induced depression of Ca(2+)-dependent K+ channels (KCa channels) in network neurons. 2. The actions of the spinal 5-HT system were analyzed in the lamprey spinal cord preparation in vitro. Fictive swimming was induced by bath application of N-methyl-D-aspartate (NMDA). The intersegmental phase lag between ventral root burst activities was measured along the ipsilateral side of the spinal cord. The chamber with the preparation was partitioned into two pools so that the rostral and caudal halves of the preparation could be perfused independently with solutions containing the same level of NMDA (100-150 microM) with or without additional 5-HT or a 5-HT uptake blocker (citalopram). 3. Addition of 5-HT to one of these partitioned pools changed the intersegmental phase lag in this pool, whereas the cycle duration remained unchanged. It was determined by the activity in the "non-5-HT" pool. Addition of 5-HT to the caudal pool resulted in an increased rostrocaudal phase lag. When 5-HT was added to the rostral pool, on the other hand, the phase lag shifted direction to a backward coordination.(ABSTRACT TRUNCATED AT 250 WORDS)