We have reported in our previous studies that stimulation of the spla- nchnic nerve evoked unit discharges in thalamic PO nucleus of cat which showed the characteristics of pain discharge. The present paper further stu- died the relationship between A-fiber afferent impulse of splanchnic nerve and the evoked discharges of PO nucleus. Experments were carriedout on 11 cats immobilized witb flaxedil under artificial respiration. Unit discharges were recorded by glass microelectrode (1-2μ) from PO nucleus and the afferent impules from splanchnic nerve were monitored by recording its action potentials. 25 units were found to show evoked discharges by stimulating the spla- nchnic nerve with a strong single pulse. The latent period was 58±12.1 msec. weak stimulation of splanchnic were (only A. fibers were activated)fa- iled to evoke any discharge in PO. Activation of all A fibers (especially the Aδ fibers) by stronger sti- mulation evoked discharges in PO nucleus. Blockade of A fibers by direct current eliminated these evoked discha- rges. The spontaneous discharge of all the 25 units decreased or disappeared following the evoked discharges

and then restored or even angmented. The results indicated that Aδ activation was responsible for the evoked cluster of discharges with short latent period by afferent splanchnic nerve impu- lses. The evoked discharges were followed by an inhibitory period during which the spontaneous discharges were inhibited.