In order to shed some light on the neurotransmitters in the spinothalamic tract (STT), we examined, biochemically and immunohistochemically, the contents of various neurotransmitter candidates in the terminal field of the STT after cervical hemi-chordotomy (HC) and dorsal quadrant-chordotomy (dQC) in the rat. Substance P (SP), calcitonin gene-related peptide (CGRP), enkephalin, neuropeptide Y, neurotensin, oxytocin and dynorphin A were analyzed immunohistochemically. The contents of neuropeptides (SP, CGRP and cholecystokinin octapeptide) were measured by radioimmunoassay and those of amino acids (aspartic acid, glutamic acid, gamma-aminobutyric acid (GABA) and glycine) and noradrenaline were determined using high-performance liquid chromatography. Cervical hemi-chordotomy, but not dQC, caused significant decreases of the SP-like immunoreactivity in and SP content of the ventral thalamus on the ipsilateral side, compared with that on the contralateral side and of rats subjected to sham-operation. However, neither HC nor dQC resulted in any changes in the ventral thalamic contents of other putative neurotransmitters examined. These results suggest that, in rats, the STT contains SP and that SP-positive fibers run in the ventral half of the ascending spinal tract at the cervical level.