The organization of the spinal projection to the lateral reticular nucleus (LRN) in the rat was investigated by means of a retrograde pathway tracing method in which horseradish peroxidase (HRP) served as an enzyme marker. Discrete placements of HRP into the LRN were achieved by a combination of dorsal (stereotaxic) and ventral (microsurgical) approaches. The extent and distribution of retrogradely labelled neurons in the spinal cord indicated a substantial and highly ordered projection to the LRN. All segments of the spinal cord contributed to the projection to the LRN. The limb enlargements were the riches sources of projection. The projection from the cervical spinal cord was bilateral with ipsilateral preponderance. The cervical neurons projecting to the ipsilateral LRN were located mainly in lamina VII of the spinal cord, while those projecting to the contralateral LRN were located mainly in lamina VIII. The lumbar spinal projection terminated in the contralateral LRN, and the neurons of the origin were located mainly in lamina VIII. Most if not all parts of the LRN received afferents from the spinal cord. The projections terminated most abundantly in the caudomedial portions of the magnocellular LRN. The medial aspect of the LRN was the site of preferential termination of the cervical spinal projection. Lumbar spinal projections terminated preferentially in the rostrolateral region of the nucleus. These two imputs overlap in the central region of the nucleus.