The distribution of nerve terminal-like structures (herein called nerve terminals), fibers, and neurons containing neuropeptide Y-like immunoreactivity (NPY-ir) was studied immunohistochemically in cat spinal cord with and without colchicine treatment. Rexed laminae II and III of the dorsal horn contained large amounts of immunoreactive nerve terminals and few fibers at all levels of the cord whereas laminae I and IV-VI contained fewer terminals and numerous fibers. In segments C7-T3, fibers with NPY-ir in the superficial laminae collected into bundles which travelled ventromedially toward the dorsal gray commissure (DGC). In addition, another bundle of fibers was present in segments C8-T2 and T11-S2; these fibers also originated from the upper dorsal laminae and travelled along the dorsomedial border of the gray matter to cross the midline in the DGC. In the intermediate and central gray, most immunoreactivity was found in the autonomic areas: terminals and fibers containing NPY-ir were found in the intermediolateral cell column pars principalis (IMLp) in all segments between C8 and L4 with the densest accumulation in segments T6 and T7. All other autonomic areas contained immunoreactive structures in nearly all thoracolumbar segments except for the IML pars funicularis, which contained small numbers of immunoreactive fibers only between segments T2 and T8, inclusive. In the sacral cord, the autonomic areas in the intermediate and central gray also contained relatively large numbers of immunoreactive terminals and fibers.(ABSTRACT TRUNCATED AT 250 WORDS)