The rat L5/6 disc is innervated from T13 to L6 dorsal root ganglia (DRGs) multisegmentally. Sensory fibers from T13, L1 and L2 DRGs have been reported to innervate through the paravertebral sympathetic trunks, whereas those from L3 to L6 DRGs innervate directly through sinuvertebral nerves on the posterior longitudinal ligament (PLL). The presence of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive (ir) nerve fibers has been demonstrated in the lumbar intervertebral discs, but their percentages in DRG neurons have not been studied. Fluoro-gold (F-G) labeled neurons innervating the L5/6 disc were distributed throughout DRGs from T13 to L6 levels. Of F-G labeled neurons innervating the L5/6 disc, the percentage of SP-ir T13 to L6 DRG neurons was 30%, and that of CGRP-ir neurons was 47%. The mean cross-sectional area of the cell of SP-ir neurons was 696+/-66 microm2 (mean +/- S. E.), and that of CGRP-ir neurons was 695+/-72 microm2 (mean +/- S. E.). SP- and CGRP-ir were mainly observed in small neurons. The percentages of SP- or CGRP-ir neurons in L1 and L2 DRGs innervating the L5/6 disc were not different from those in L3, L4 or L5 DRGs. In the physiological condition in rats, DRG neurons at all levels may have the same significant role in pain sensation of the disc.