In Nembutal-anesthetized and artificially ventilated cats, we studied the morphological properties of the inspiratory neurons of the dorsal respiratory group (DRG) with HRP intracellular staining. A total of 37 neurons were stained and their axonal trajectories and terminal distribution in the medulla were analyzed. The somata were located predominantly in the ventrolateral region of the nucleus of the solitary tract and were distributed between 2,300 mum rostral and 700 mum caudal to the obex. Most (26/33) of the neurons tested were antidromically activated by the stimulation of the contralateral (n = 24) or ipsilateral (n = 2) cervical cord. To examine the existence of collateral branches in the brainstem, we traced axonal trajectories in 28 neurons. In most cases, the stem axons issuing from the cells of origin coursed ventrally and then turned medially to cross the midline without giving off any axon collaterals. However, six neurons had axonal collaterals in the brain stem ipsilateral to the somata. At least four types of collateralization were observed. The stem axon of the first type bifurcated at the area ipsilateral and ventral to the cell body. One branch crossed the midline to project to the spinal cord, and the other, thinner branch descended caudally in the ipsilateral medullary reticular formation without distributing any terminals. The axon of the second type projected to the contralateral spinal cord and distributed collateral branches with terminal boutons in the ipsilateral ventral respiratory group (VRG). The third type projected to the contralateral spinal cord and distributed terminal boutons in the medial part of the nucleus of the solitary tract (NTS) and its vicinity. The fourth type distributed numerous branches with terminal boutons in and around the ventral part of the NTS and the VRG area. This study indicates that some inspiratory neurons of the DRG influence not only spinal respiratory neurons but also medullary respiratory neurons in the vicinity of the DRG and the VRG.