The in vitro lamprey spinal cord contains a "central pattern generator" (CPG) that can generate locomotor activity with excitatory amino acids added to the bath. The motor pattern can be entrained by imposed rhythmic bending of either the caudal or rostral end of the notochord/spinal cord. In the present study, the quantitative and mechanistic features of entrainment were investigated. Increasing the amplitude of the imposed movement increased the range of frequencies over which entrainment occurred. Brief, pulsed, imposed movements could reset the locomotor rhythm. During entrainment at different imposed movement frequencies, the burst duration was a constant proportion of about 35% of the cycle time. The intersegmental phase lag, which is usually constant at about 0.01 during locomotion, decreased significantly with caudal imposed movements. A small increase in the phase lags was observed with rostral imposed movements. In low-calcium Ringer's, briefly bending the notochord/spinal cord activated intraspinal mechanoreceptors and elicited ascending and descending unit activity in lateral spinal fascicles many segments from the point of bending. However, this ascending or descending movement-related activity was insufficient to fully entrain the locomotor rhythm, since blocking the pattern in the region of bending abolished 1:1 entrainment. The mechanoreceptors appear to act locally on the CPG networks, since interrupting the ascending or descending movement-related activity with lesions of the lateral fascicles did not abolish entrainment. In contrast, stripping the very lateral margins of the spinal cord in the region of bending did abolish entrainment, presumably by destroying the transduction region of the mechanoreceptors. The data, taken together, suggest that the mechanoreceptors entrain the local CPG networks, and this timing information is then distributed to the other parts of the spinal motor networks through the coordinating system.