Abstract Slow voltage-sensitive dyes work by accumulating in brain tissue and report the average membrane potential of neurons and glia. The voltage-sensitive dye diO-C2-5 was used to monitor the polarization state of 27 brain structures in the rat during a systemically induced, behaviorally mild, kainic acid seizure using a 20 s recording period. The effects of the anesthetic agent used in the experiment were minimized by delaying the dye injection and seizure mapping for one day. Eleven areas were depolarized during the seizure, but 16 other areas did not change their polarization state compared to controls. The effects of pentobarbital appear to have no measurable effect on seizure propagation once the animal has behaviorally recovered from the anesthesia. The technique allows for mapping areas of seizure involvement with a unique combination of spatial and temporal resolution.