The intracellular activity of motoneurons during 'fictive' respiration in the isolated lamprey brain was investigated. In association with each respiratory cycle three distinct PSP phases were observed: an early, low amplitude EPSP phase; a large, brief EPSP phase that drove action potentials; and a subsequent IPSP phase. Selective midline and trigeminal lesions, and trigeminal stimulation, demonstrated that the large excitatory and inhibitory phases were generated by a previously described pair of central pattern generators located in the trigeminal region of the medulla. Lesion studies further showed that the low amplitude excitatory input could be produced independently of the trigeminal pacemakers, near the region of the medulla that contains the respiratory motoneurons. In addition to 'normal' fictive respiration, the isolated brain was found to produce several variations of the respiratory pattern. These motor programs, 'coughs', 'arousal breathing', and 'weak breathing', were distinguished from the normal respiratory pattern by their much longer burst durations, distinctive underlying synaptic input, and separate coordinating mechanism. Activity similar to these motor programs could be independently produced by the caudal medulla after both trigeminal central pattern generators had been removed. Lesion studies, and the observation that respiratory-related neural activity ceased in the trigeminal region during the production of these long-duration programs, suggest that the caudal medulla also contains paired central pattern generators involved in lamprey respiration.