1. As part of a continuing analysis of the mechanisms of the central pattern generator underlying fictive swimming in lamprey, a systematic survey of electrophysiological properties of lamprey neurons was made in the in vitro spinal cord preparation with the use of intracellular current-clamp recordings. A total of 70 neurons was included in the study, representing 6 classes of spinal neurons. The classes were myotomal motoneurons, three classes of interneurons involved in fictive swimming [lateral interneurons, nerve cells with contralateral and caudal projecting axons (CC interneurons), and excitatory interneurons], and two classes of interneurons involved in sensory processes (edge cells and giant interneurons). The recordings were done in quiescent preparations. 2. There was little or no significant difference among the cell classes with regard to resting potential, threshold potential, action-potential amplitude, or action-potential duration. 3. The voltage versus current relationships for the cells were fairly linear near resting potential, although most cells showed a slight tendency to rectify with depolarization above resting potential. This tendency was strongest among edge cells and lateral interneurons and weakest among motoneurons and CC interneurons. The input resistances, membrane time constants, and rheo-bases for the cell classes showed significant differences among some classes. For example, CC interneurons and excitatory interneurons had significantly higher input resistances than the other cell classes. 4. The late afterhyperpolarization following the action potential tended to be larger in amplitude with an earlier peak and a longer duration in edge cells and giant interneurons than in the other cell classes. 5. All cells responded to depolarizing current injections by firing action potentials, and almost all cells fired action potentials throughout the 400-ms current pulse. The cells exhibited adaptation resulting in increasing interspike intervals during the current pulse. The adaptation, however, was insufficient to terminate firing before the end of the current pulse. The relationship between frequency of firing and input current was generally monotonic with a tendency to saturate at higher current levels. 6. The general conclusion from this study is that the spinal neurons that partake in fictive swimming (motoneurons, lateral interneurons, CC interneurons, and excitatory interneurons) are similar in their resting and action-potential mechanisms. Their most prominent differences are in size-related properties. The sensory-related interneurons, especially the edge cells and to some extent the giant interneurons, exhibited more pronounced differences in their resting and action-potential properties when compared with the other cell classes.