Whole body hyperthermia (WBH) has been used in experimental settings as an adjunct to radio-chemotherapy for the treatment of various malignant diseases. The therapeutic effect of WBH has been hypothesized to involve activation of the immune system, although the effect of hyperthermia-induced activation of sympathetic nerve discharge (SND) on splenic immune function is not known. We tested the hypothesis that heating-induced splenic sympathoexcitation would alter splenic cytokine gene expression as determined using gene array and real-time RT-PCR analyses. Experiments were performed in splenic-intact and splenic-denervated anesthetized Sprague-Dawley rats (n = 32). Splenic SND was increased during heating (internal temperature increased from 38° to 41°C) in splenic-intact rats but remained unchanged in nonheated splenic-intact rats. Splenic interleukin-1 (IL-1), interleukin-6 (IL-6), and growth regulated oncogene 1 (GRO 1) mRNA expression was higher in heated than in nonheated splenic-intact rats. Splenic IL-1, IL-6, and GRO 1 mRNA expression was reduced in heated splenic-denervated compared with heated splenic-intact rats, but did not differ between heated splenic-denervated and nonheated splenic-intact rats. These results support the hypothesis that hyperthermia-induced activation of splenic SND enhances splenic cytokine gene expression.