The involvement of the claustrum in processing of somatosensory information was investigated using the formalin test as an established pain test. The metabolic activity of the claustrum and other brain structures belonging to the somatosensory system was assessed by autoradiographic method of the uptake of (3H)-labelled 2-deoxyglucose (2-DG). Glucose utilization in the claustrum was stimulated by subcutaneous injection of formalin into the front paw. The increase of radioactivity was observed not only in the claustrum, but also in the anterior pretectum, the ventral thalamic nucleus, the somatosensory, insular and piriform cortices on the contralateral side to formalin stimulation. After stereotactical injection of morphine into the contralateral pretectum to stimulation side, uptake of 2-DG in the claustrum was abolished. These results indicate that the claustrum is an important relay nucleus in the somatosensory system, which may convey somatosensory information from the pretectum to the cerebral cortex, thus by-passing the thalamus, throughout the pretecto-claustro-cortical pathways.