1. Application of D-glutamate to the isolated spinal cord of the lamprey produces phasic activity in ventral roots, which is similar to that of the muscles of the intact swimming animal (5,18). Therefore, the isolated spinal cord may be used as a convenient model for the investigation of the generation of locomotor rhythms in a vertebrate. 2. Almost all slow muscle fibers exhibited excitatory junctional potentials (EJPs) during swimming activity. The number of EJPs per cycle increased with the intensity of ventral root (VR) bursting. Few twitch fibers were active, and these fired action potentials only during high intensities of VR bursts. 3. As was found by Russell and Wallén (25), myotomal motoneurons had oscillating membrane potentials during fictive swimming which, on the average, reached a peak depolarization in the middle of the VR burst (phi = 0.21 +/- 0.05; phi = 0 is defined as the onset of the VR burst, and the duration of the cycle is set equal to 1). Membrane potential oscillations in fin motoneurons were antiphasic to those of nearby myotomal motoneurons (peak depolarization phi = 0.68 +/- 0.05). 4. Lateral interneurons had oscillating membrane potentials in synchrony with those of myotomal motoneurons (peak depolarization phi = 0.21 +/- 0.10). Interneurons with axons projecting contralaterally and caudally (CC interneurons) had oscillating membrane potentials that peaked significantly earlier in the cycle (peak depolarization phi = 0.06 +/- 0.12). 5. Edge cells were only weakly modulated during fictive swimming. Their peak depolarizations occurred near the end of the VR burst (phi = 0.33 +/- 0.10). Most giant interneurons were not phasically modulated during fictive swimming. 6. Repetitive intracellular stimulation of Müller cells during fictive swimming generally evoked an increased burst intensity in ipsilateral VRs and a decreased burst intensity in contralateral VRs. The cells M3, B1, and B2 also produced increases or decreases in the frequency of VR bursts. Repetitive intracellular stimulation of sensory dorsal cells could also change the intensities and timing of VR bursts. 7. This study is an initial survey of lamprey spinal interneurons that participate in swimming activity. Lateral interneurons and CC interneurons are active during fictive swimming and probably help coordinate the undulations of the body, but their roles in pattern generation are not known. The central pattern generator is subject to modification by descending and sensory inputs.