It is known that the splenic natural killer cell (NK) activity 4 weeks after unilateral resection of the cervical sympathetic nerve (csn) is significantly lower than that after sham operation in rats. To investigate the role of the splenic sympathetic nerve, the splenic NK activity after csn was compared in the control animals (n = 9) and the splenic sympathetic nerve denervated animals (n = 8). The splenic NK activity in control group was reduced significantly after csn, whereas that in splenic nerve denervated group did not reveal significant change after csn. The results suggest that the reduction of the splenic NK activity after csn is induced through the modulation of the splenic sympathetic nerve activities.