The role of neurokinin 1 (NK₁) receptor and possible interaction between NK₁ and N-methyl-D-aspartic acid (NMDA) glutamatergic receptors were investigated on spinal c-fos expression after lower urinary tract irritation with acetic acid infusion in rats. At both levels of the first (L₁) and sixth lumbar (L₆) spinal cord, where most of hypogastric nerve and pelvic nerve afferent terminals project, respectively, the selective NK₁ receptor antagonist CP-99,994 dose dependently reduced the total number of c-fos protein (Fos)-positive cells. However, CP-100,263, the enantiomer of CP-99,994 with a very low affinity for NK₁ receptor, did not have any effect on the total number of Fos-positive cells. Coadministration of a low dose (1 mg/kg) of CP-99,994 and NMDA receptor antagonist (MK-801), either of which alone did not affect c-fos expression, significantly inhibited c-fos expression at both levels of the spinal cord. Regarding regional differences, the number of Fos-positive cells decreased significantly at all regions of the L₆ level, but only at the dorsal horn of the L₁ level. These results indicate that NK₁ receptor is involved in spinal c-fos expression after lower urinary tract irritation and that NK₁ and NMDA receptors have a synergistic interaction in the spinal processing of nociceptive input from the lower urinary tract.