Changes in the expression of dynorphin were investigated immunohistochemically. Cells showing dynorphin immunoreactivity first appeared 2 days and disappeared 15 days after the unilateral insertion of separating elastics between the upper molars. These immunopositive cells were localized in the superficial layers in the medial third of the subnucleus caudalis on the ipsilateral side, except at 1 and 15 days. On the contralateral side, a few immunoreactive cells only were observed in the subnucleus caudalis of the 3- to 8-day groups. No immunoreactivity was observed in the subnucleus interpolaris, subnucleus oralis spinal trigeminal nucleus complex (STNC), principal trigeminal nucleus and mesencephalic trigeminal nucleus of the experimental rats and all the trigeminal nuclei of control animals. The findings indicate that experimental tooth movement induced the expression of dynorphin in the superficial layers of the subnucleus caudalis STNC. The subnucleus caudalis may play an important part in modulation of the discomfort and pain evoked by tooth movement.