Respiration requires the coordinated rhythmic contractions of diverse muscles to produce ventilatory movements adapted to organismal requirements. During fast locomotion, locomotory and respiratory movements are coordinated to reduce mechanical conflict between these functions. Using semi-isolated and brainstem-spinal cord preparations from neonatal rats, we have characterized for the first time the respiratory patterns of all spinal intercostal and abdominal motoneurons, and explored their functional relationship with limb sensory inputs. Neuroanatomical and electrophysiological procedures were initially used to locate intercostal and abdominal motoneurons in the cord. Intercostal motoneuron somata are distributed rostro-caudally from C7-T13 segments. Abdominal motoneuron somata lie between T8 and L2. In accordance with their soma distributions, inspiratory intercostal motoneurons are recruited in a rostro-caudal sequence during each respiratory cycle. Abdominal motoneurons express expiratory-related discharge that alternates with inspiration. Lesioning experiments confirmed the pontine origin of this expiratory activity which was abolished by a brainstem transection at the rostral boundary of the VII nucleus, a critical area for respiratory rhythmogenesis. Entrainment of fictive respiratory rhythmicity in intercostal and abdominal motoneurons was elicited by periodic low-threshold dorsal root stimulation at lumbar (L2) or cervical (C7) levels. These effects are mediated by direct ascending fibers to the respiratory centers and a combination of long-projection and polysynaptic descending pathways. Therefore, the isolated brainstem-spinal cord in vitro generates a complex pattern of respiratory activity in which alternating inspiratory and expiratory discharge occurs in functionally identified spinal motoneuron pools that are in turn targeted by both forelimb and hindlimb somatic afferents to promote locomotor-respiratory coupling.