Axonal projections of eighty-four rostral medullary expiratory neurones of the Bötzinger complex were tested using antidromic mapping techniques in anaesthetized cats. A projection to the ventral respiratory neurones of the medulla (n.r.a.) was shown in eleven out of twelve tested neurones. Also a spinal projection to the C5-C6 cervical segments was evident in more than 72% of tested neurones; probably near 100% project to cervical cord. These axonal projections were found bilaterally in both brain stem and spinal cord. The majority of Bötzinger complex expiratory neurones were seen to have two to four axonal collaterals to the ventro-lateral (v.l.) nucleus of the solitary tract (n.t.s.) and/or the n.r.a. and/or the spinal cord. In eight out of twelve of the tested neurones, electrophysiological evidence of axonal arborization in more than one of n.r.a. inspiratory, n.r.a. expiratory or v.l. n.t.s. regions was obtained. Similar evidence for the terminal arborization was found for 26% of tested neurones in the phrenic motor nucleus. The descending spinal expiratory axons of the Bötzinger complex neurones are located in the dorsal and medial parts of the lateral funiculus in C4 and C5 segments. Conduction velocity measurements indicate that these are large myelinated axons. We propose that the Bötzinger complex expiratory neurones are a source of synaptic inhibition for n.r.a. inspiratory neurones and phrenic motoneurones.