The present study was carried out to characterize the functional properties of spinomesencephalic tract (SMT) neurons in the lumbar spinal cord of urethane anesthetized rats. Extracellular single unit recordings were made from neurons antidromically activated by stimulation of the midbrain area, including the deep layers of superior colliculus, periaqueductal gray and midbrain reticular formation. Recording sites were located in laminae I-VII of spinal cord segments of L2-L5. Receptive field properties and responses to calibrated mechanical stimulation were studied in 78 SMT cells. Mean conduction velocity of SMT neurons was 19.1±1.04 m/sec. SMT units were classified according to their response profiles into four groups: wide dynamic range (58%), deep/tap (23%), high threshold (9%) and low threshold (3%). A simple excitatory receptive field was found for most SMT neurons recorded in superficial dorsal horn (SDH). Large complex inhibitory and/or excitatory receptive fields were found for cells in lateral reticulated area which usually showed long after-discharge. Most of SMT cells received inputs from Aδ and C afferent fiber types. These results suggest that sensory neurons in the rat SMT may have different functional roles according to their location in the spinal cord in integrating and processing sensory inputs including noxious mechanical stimuli.