Spontaneous high-frequency, low-amplitude and low-frequency, high-amplitude efferent bursting patterns of cranial and spinal motor nerve activity in the in vitro brainstem preparation of the bullfrog tadpole Rana catesbeiana have been characterized as fictive gill and lung ventilation, respectively (Gdovin MJ, Torgerson CS, Remmers JE). Characterization of gill and lung ventilatory activity in cranial nerves in the spontaneously breathing tadpole Rana catesbeiana, FASEB J 1996;10(3):A642; Gdovin MJ, Torgerson CS, Remmers JE. Neurorespiratory pattern of gill and lung ventilation in the decerebrate spontaneously breathing tadpole, Respir Physiol 1998;113:135 146; Pack AI, Galante RJ, Walker RE, Kubin LK, Fishman AP. Comparative approach to neural control of respiration, In: Speck DF, Dekin MS, Revelette WR, Frazier DT, editors. Respiratory Control Central and Peripheral Mechanisms. Lexington: University of Kentucky Press, 1993:52-57). In addition, the ontogenetic dependence of central respiratory chemoreceptor stimulation on fictive gill and lung ventilation has been previously described (Torgerson CS, Gdovin MJ, Remmers JE. Fictive gill and lung ventilation in the pre- and post-metamorphic tadpole brainstem, J Neurophysiol 1998, in press). To investigate the neural substrates responsible for central respiratory rhythm generation of gill and lung ventilation in the developing tadpole, we recorded efferent activities of cranial nerve (CN) V, VII, and X and spinal nerve (SN) II during changes in superfusate PCO2 before and after multiple transection of the in vitro brainstem. The brainstem was transected between CN VIII and IX and the response to changes in PCO2 was recorded. A second transection was then made between the caudal margin of CN X and rostral to SN II. Preliminary data reveal that robust gill ventilation was recorded consistently only if the segment of brainstem included CN X, whereas the loci capable of eliciting fictive lung bursting patterns appeared to differ depending on developmental stage. These data demonstrate that the neural substrate required for fictive gill and lung ventilation exists in anatomically separate regions such that the gill central pattern generator (CPG) is located in the caudal medulla at the level of CN X throughout development, whereas the location of the lung CPG is located more rostrally at the level of CN VII in the post-metamorphic larva. Both in vivo and in vitro studies revealed two distinct neural bursting patterns associated with gill and lung ventilation. Sequential activation of CN V, VII, X were observed during gill ventilation of in vivo and fictive gill ventilation in vitro, whereas these nerve activities, along with SN II displayed more synchronous bursting patterns of activation during lung ventilation and fictive lung breaths.