Static contraction of hindlimb skeletal muscle in cats induces a reflex pressor response. The superficial dorsal horn of the spinal cord is the major site of the first synapse of this reflex. In this study, static contraction of the triceps surae muscle was evoked by electrical stimulation of the tibital nerve for two minutes in anesthetized cats (stimulus parameters: 2 times motor threshold at 30 Hz, 0.025 ms duration). Ten stimulations were performed and one-minute rest was allowed between stimulations. Muscle contraction caused a maximal increase of 32±5 mm Hg in mean arterial pressure (MAP), which was obtained from the first three contractions. Activated neurons in the superficial dorsal horn were identified by Fos protein. Distinct Fos expression was present in the L6-S1 level of the superficial dorsal horn ipsilateral to the contracting leg (88±14 labeled cells/section at the L7); whereas, only scattered Fos expression was observed in the contralateral superficial dorsal horn (9±2 labeled cells/section, p<0.05 compared with ipsilateral section). A few Fos labeled cells were found in control animals (12±5 labeled cells/section, p<0.05 compared with stimulated cats). Furthermore, double-labeling methods demonstrated that Fos protein co-existed with nitric oxide synthase (NOS) positive staining in the superficial dorsal horn. Finally, intrathecal injection of an inhibitor of NOS, L-NAME (5 mM), resulted in fewer Fos labeled cells (58±12 labeled cells/section) and a reduced maximal MAP response (20±3 mm Hg, p<0.05). These results suggest that the exercise pressor reflex induced by static contraction is medicated by activation of neurons in the superficial dorsal horn, and that formation of nitric oxide in this region is involved in modulating the activated neurons and the pressor response to contraction.