By use of specific antisera, the distributions of immunoreactive dynorphin (ir-DYN), alpha-neo-endorphin (ir-alpha-NEO), Met-enkephalin (ir-MET) and substance P (ir-SP) were evaluated in discrete regions of human spinal cord and spinal ganglia. The relative concentrations of immunoreactive peptides in particular regions were as follows: sacral greater than lumbar greater than cervical greater than thoracic. Concentrations of ir-DYN, ir-alpha-NEO and ir-SP were 2-10-fold, but of ir-MET 1-2-fold, higher in the dorsal as compared to the ventral parts of cervical, lumbar and sacral cord. The concentrations of all peptides (when examined in discrete areas of thoracic cord) were found to be highest in the substantia gelatinosa. All peptides were present in the gray matter but only ir-MET was found in white matter. Gel-permeation chromatography of dorsal sacral spinal cord extracts revealed two major ir-DYN peaks. The smaller molecular weight peak, eluted at the position of synthetic dynorphin1-17. ir-alpha-NEO and ir-SP comigrated exactly with their respective synthetic marker peptides. Substantial amounts of ir-SP and also, as confirmed by high pressure liquid chromatography, ir-MET, were found in the dorsal and ventral roots and spinal ganglia, and very low concentrations of ir-DYN or ir-alpha-NEO were also detected in these tissue. These results suggest that dynorphin and alpha-neo-endorphin, in addition to enkephalins, may be involved in transmission of somatosensory information in the human spinal cord.