We report on the optical imaging of spontaneous respiratory neuron bursts in the ventrolateral medulla (VLM) of medullary slices or brain stem-spinal cord preparations. A medullary slice with a thickness of 1.0-1.4 mm or brain stem-spinal cord from 0- to 4-d-old rats was stained with fluorescent voltage-sensitive dye, RH795. Optical signals were recorded as a fluorescence change by using an optical recording apparatus with a 128 x 128 photodiode array and a maximum time resolution of 0.6 ms. Motoneuronal activity was simultaneously recorded at the hypoglossal nerve roots or fourth cervical ventral roots. Fluorescence changes corresponding to the spontaneous inspiratory burst activity were detected in the hypoglossal nucleus and VLM in slice preparations, and in a limited area extending rostrocaudally in the VLM of the brain stem-spinal cord preparation. These measurements did not require signal averaging by multiple trials. Results suggest that inspiratory neurons are localized in more compact form at the level of the nucleus ambiguous than at the more rostral VLM, and that peak activity during the inspiratory phase propagates from the caudal to the rostral VLM. In 60% of brain stem-spinal cord preparations, weak and scattered fluorescence changes preceding the inspiratory burst activity were detected more predominantly in the rostral part of the VLM. The present findings show the feasibility of optical recordings for the in vitro analysis of spontaneous respiratory neuron activity in the medulla.