Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (= 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.