In experimental jejunal obstruction produced in cats, the neurones of the intermediate gray column of the cord showed bilateral intranuclear inclusions. They were also seen when hypertrophy and hyperplasia, produced in various elements of the bowel wall, were prevented by large doses of atropine. The latter observation implied that the afferent autonomic nerve supply is involved in the production of these unusual structures. The inclusion bodies were extremely rare or absent in control animals. In order to see whether these bodies occurred by chance even though they are regularly produced in the above-mentioned area, an effort was made to see whether they would appear unilaterally following stimulation of a unilateral afferent nerve. This was done by ligation and severance of the right common carotid artery below its bifurcation with or without severing its accompanying vagal trunk. Removal of the pulsatile blood supply to the carotid sinus and carotid body has long been known to produce afferent action potentials in the glossopharyngeal and vagus nerves. Within 6 h many neurones in the middle half of the ipsilateral nucleus of the tractus solitarius showed the inclusions. The cephalic end of this nucleus showed no inclusions. At the level of the obex the inclusions were present bilaterally. The inclusions also appeared in numerous other areas of the brain, brain stem and cervical cord in bilateral fashion. They were found in neurones of the neurohyphysis as well as in the cingulate gyrus. Their widespread but focal distribution corresponds to numerous areas of the cord and brain in which physiologists have produced striking effects with electric probing and stimulation. The inclusion bodies are at present conceived as representing the neuronal nuclear response to an afferent stimulus which causes an increase in the metabolic activity controlled by the nucleus.