Immunofluorescence histochemistry was used to determine the distribution of substance P, somatostatin and cholecystokinin-octapeptide-immunoreactive perikarya in C6, T6, T10, L2 and S1 dorsal root ganglia of rat. Five different categories of immunoreactive primary afferent neurons were distinguished on the basis of cell size, cytology and peptide immunoreactivities. The population of small cells (diameter <20 μm) included three groups which were identified as containing somatostatin, substance P, or substance P + cholecystokinin-octapeptide. Two groups of cells were identified in an intermediate size range (diameter 21-43 μm) as containing cholecystokinin-octapeptide or cholecystokininoctapeptide + substance P. These categories may reflect four distinct populations of primary afferent neurons. The relative abundance of dorsal root ganglion cells containing substance P, cholecystokininoctapeptide or somatostatin immunoreactivities was significantly different within segmental levels. More neurons were immunoreactive for cholecystokinin-octapeptide than substance P in ganglia C6, T6 and T10. Somatostatin-containing cells were fewest in number regardless of level. The number of immunoreactive cells also varied among spinal ganglia. L2 contained the greatest number of immunoreactive cells; S1 contained the fewest. These studies are relevant to our understanding of dorsal root ganglia in two ways. Firstly, the data document significant variation in the distribution of peptide-containing neurons among spinal ganglia associated with various cord levels. The variation in peptide-containing cell populations among spinal ganglia may reflect differences in populations of modality-specific primary afferent fibers as well as in populations of somatic and visceral primary afferent fibers at each level. Furthermore, the data indicate that the relative abundance of a population of peptide-containing primary afferent neurons cannot be extrapolated from the examination of spinal ganglia from a single level. Secondly, substance P and cholecystokinin-octapeptide did not co-exist in all spinal ganglion cells as previously reported. In conjunction with immunostaining characteristics and cell size, the differential distribution of the two peptides defined four cell types, raising the possibility that each cell type may mediate a different modality.