An in vitro preparation of the lamprey spinal cord was developed to enable detailed studies of the neuronal organization of the central spinal network generating fish swimming movements, one basic type of vertebrate locomotion. 1. In the isolated lamprey spinal cord, stable bursting activity recorded in the ventral roots was initiated by adding, e.g., D-glutamate or L-DOPA to the bathing solution. Less stable rhythmic activity could also be induced by tonic electrical stimulation of the spinal cord. 2. The isolated spinal cord is capable of producing rhythmic activity with the same type of intra- and intersegmental coordination as in the swimming fish, i.e., with alternation between the two sides of the segment and and intersegmental phase coupling. Hence, the in vitro preparation of the lamprey spinal cord may be said to represent the neuronal correlate of the undulatory swimming movements of fish. 3. By performing spinal cord transections it was demonstrated that as few as four segments can produce rhythmic activity with maintained coordination. It was concluded that the capacity to produce coordinated burst activity is distributed throughout the lamprey spinal cord. 4. A longitudinal midline lesion as long as four segments did not prevent the ventral roots on each side from bursting with maintained coordination between adjacent hemisegments. Thus, one side of a segment can produce bursting without interaction with its opposite side, at least when connected to its rostral and caudal neighbors. 5. The rate of bursting was found to vary from one cycle to the next with the period length tending to oscillate about a mean value. Burst duration and intersegmental phase lag varied in the same manner.