This study investigated the interaction between vagal afferent input and central chemosensitivity in modulating the respiratory motor output of in vitro brain stem-spinal cord preparations from adult bullfrogs. With this preparation, the spatiotemporal distribution of respiratory-related motor output emulated that of intact bullfrogs; that is, the fictive breathing pattern was mostly episodic. Recordings from cranial motor nerves (V and X) showed that, without peripheral feedback, increasing the PCO2 of the mock cerebrospinal fluid (thereby reducing pH from 8.3 to 7.7) caused a modest increase in respiration-related burst frequency. When the pulmonary branch of a vagus nerve was stimulated phasically (2 V, 20 Hz, 0.2 ms) during each fictive breath to simulate afferent pulmonary stretch receptor feedback 1) the responsiveness of the preparation to the same changes in pH was augmented nearly threefold and 2) the breathing pattern remained episodic. It appears, therefore, that episodic breathing is an intrinsic property of the central nervous system in bullfrogs. It is concluded that there is a strong interaction between vagal feedback and central chemodetection in controlling the temporal relationships that characterize this episodic breathing pattern.