Spatial dynamic patternings of electrical responses to vagal nerve stimulations in the embryonic chick brainstem preparation were assessed by means of simultaneous multiple-site optical recordings of electrical activity. The vagus/brainstem preparations were dissected from early 7-day-old chick embryos (165-172 h after incubation), and they were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Application of depolarizing square current pulses to the vagus nerve fibers using a suction electrode evoked voltage-related optical (absorbance) signals that were recorded simultaneously from 127 contiguous sites in the whole brainstem preparation using a 12 x 12-element photodiode array. The optical signals evoked by the vagus nerve stimulation appeared to be concentrated longitudinally in the central region and in the lateral region ipsilateral to the site of brainstem stimulation. These response areas were orderly changed according to changes in the strength and in the duration of the stimulating current: the response area expanded as the strength or the duration of the stimulating current was increased. The size of the evoked optical signals also depended on the strength and on the duration of the stimulating current. We also measured critical (threshold) values of the strength and duration of the stimulating current to produce the optical responses, and we found that there was a regionally ordered distribution in the brainstem. In the experiments on transverse slices of the brainstem, the evoked optical responses were detected from limited areas near the dorsal surface of the stimulated side, and the response pattern depended on the strength and duration of the stimulating current in a manner similar to that in the whole brainstem preparations. On the basis of the data obtained from these experiments, we have constructed 3 kinds of response maps for the embryonic brainstem, and the results suggest the functionally ordered arrangement of the neurons in the vagus-related area in the embryonic brainstem.