We describe a visual assay of neuronal electrophysiologic status for use with cultured neurons, based on the exclusion of propidium iodide (PI) by intact cellular membranes. We use this fluorescent dye, which binds to nucleic acids, at concentrations suitable for long-term exposure to neurons without toxicity. We correlate the progressive loss of resting membrane potential and the progressive inability to generate stimulated action potentials by cultured mouse dorsal root ganglion neurons with increasing incorporation of PI. The scoring system used to gauge incorporation of PI is rapid and highly reproducible using a standard fluorescence microscope. Applications exist for studies of neuronal toxicity, survival, and electrophysiology in vitro.