The present study aims to test the hypothesis that hyperbaric exposure inhibits nociceptive processing in the trigeminal spinal nucleus caudalis and C1 spinal neurons. We investigated the c-Fos-like immunoreactivity of the brainstem and upper cervical spinal cord (C1 region) following an injection of mustard oil (15 μl of 20%) into the nasal mucosa of pentobarbital anesthetized rats after exposure to hyperbaric (2-atmospheres, 1 h) and normobaric pressures. After the hyperbaric exposure, the mean number of Fos-immunoreactive neurons in the ipsilateral laminae I-II and III-IV of the trigeminal spinal nucleus caudalis were significantly lower than those in the normobaric condition. Similarly, the mean number of c-Fos positive neurons in the superficial layer (I-II) of the ipsilateral C1 segment were significantly reduced as compared with that in the normobaric condition. When treated with the vehicle alone, no significant difference was detected in the numbers of c-Fos positive neurons in the trigeminal spinal nucleus caudalis and C1 regions between hyperbaric and normobaric conditions. These results suggest that hyperbaric exposure may attenuate nociceptive signals from the area innervated by the trigeminal nerves at the level of both the trigeminal spinal nucleus caudalis and C1 dorsal horn.