1. Reliable methods for establishing fetal mouse spinal cord (SC) and dorsal root ganglion (DRG) cells in long term (greater than 1 mo) dissociated cell cultures are described. These cells have been studied by morphologic and intracellular electrophysiologic techniques. 2. Cells studied electrophysiologically can be relocated after preparation for electron microscopy and examined in thin sections. The electron microscope shows that the surface membranes of these cells were directly accessible to the culture medium. The surfaces of SC cells were studded with synaptic boutons, whereas the DRG cell surfaces generally had none. 3. Current-voltage relationships and linear electrotonic properties of the neurons are described. Delayed and anomalous rectification were seen in both cell types. The length of SC cell dendrites was about one characteristic electrotonic length, while little or no contribution of the relatively sparse DRG cell processes was seen in the transient responses of the DRG cells. 4. Postspike and posttetanic hyperpolarizations in DRG cells were due to a surface membrane conductance increase; this was probably primarily an increase in K+ conductance. Post-activation hyperpolarization in SC cells was primarily due to activation of an electrogenic Na+ pump.