The location of spinal ganglion cells projecting into various thoracic spinal nerve branches in the rat has been investigated using retrograde transport of horseradish peroxidase (HRP). In addition, total numbers of myelinated and unmyelinated axons in three consecutive pairs of thoracic spinal nerves have been analyzed. The animals were perfused one to 2 days after application of HRP to thoracic spinal nerve branches and several ipsilateral spinal ganglia sectioned and processed for HRP histochemistry. Labeled cells were found in ganglia corresponding to the level of operation, as well as several adjacent rostrally and caudally located ganglia. No somatotopic organization was found within the ganglia, but occasionally a clustering of labeled cells belonging to the same peripheral nerve was observed. Pieces from three consecutive pairs of ventral and dorsal spinal nerve rami were removed from two normal rats and embedded in Vestopal. Semithin sections including the entire cross-sectional area of the rami were used to count the total number of myelinated axons. The number of unmyelinated axons was estimated by sample counting in the electron microscope. The ventral ramus of the spinal nerve contained about twice as many axons and a lower proportion of unmyelinated fibers in comparison with the dorsal ramus. Large differences in the number of axons between the two sides in pairs of spinal nerve rami and entire spinal nerves were found. These differences diminished when comparing the total number of axons from three consecutive rami or nerves on each side.(ABSTRACT TRUNCATED AT 250 WORDS)