I studied rat lumbar intervertebral discs using a monoclonal antibody to substance P, which revealed immunoreactivity in the periosteum and ligaments adjacent to the intervertebral disc. Fibers containing substance P-like immunoreactivity were also found penetrating and terminating within the annulus fibrosus of both the anterior and posterior intervertebral disc. the maximum depth of penetration was 5 lamellae (annular rings) or approximately one sixth of the depth of the annulus. the terminal structures were not encapsulated (free-nerve endings) and were either branched, looped or both. the majority of fibers were varicose in appearance. Substance P-like immunoreactivity was very minor.