1. Electrical activity in the embryonic chick brain stem has been monitored optically. The vagus-brain stem preparations isolated from 7-day-old chick embryos were stained with voltage-sensitive merocyanine-rhodanine dyes. 2. Voltage-related optical absorption signals evoked by vagus nerve stimulation with depolarizing and hyperpolarizing pulses using a suction electrode were recorded simultaneously from 127 adjacent loci in the brain stem using a 12 x 12-element photodiode array. 3. The optical signals evoked by the stimulation appeared to be concentrated longitudinally in the central region and in the lateral region, both on the stimulated side of the brain stem, and they did not spread to the opposite side. In addition, the evoked optical responses were detected from small areas on the dorsal surface of the stimulated side, in experiments using transverse slices of brain stem. 4. The optical action potential signals evoked by the brief depolarizing stimulus were conducted slowly and were blocked completely by tetrodotoxin. With relatively long-duration depolarizing and hyperpolarizing stimulations, electrotonic responses were recorded. 5. When 2 microA/2 ms hyperpolarizing pulse stimulations were applied, anode-break excitation signals were detected, and these signals were also blocked by tetrodotoxin. 6. On the basis of the data obtained from these experiments, we constructed maps of the electrical response area and demonstrated the spatial pattern of the vagus dorsal nucleus in the 7-day-old embryonic chick brain stem.