The nervous and immune systems interact in a bidirectional fashion. For example, the neuropeptide substance P (SP) has been implicated in a variety of immune responses. Conversely, cytokines, a class of immunoregulatory glycoproteins, affect the synthesis of neurotransmitters and neurotrophic factors. This paper examines the role of cytokines in regulating neuropeptide expression in sympathetic neurons. Exposure of cultured explants of the rat superior cervical ganglion to the cytokine interleukin 1 beta (IL-1 beta) increased levels of SP. IL-1 beta increased neuronal SP expression in dissociated cultures of ganglion neuronal and nonneuronal cells but had no effect on peptide content in pure neuronal cultures. By contrast, treatment with a differentiation-promoting protein, leukemia inhibitory factor, increased SP in both pure neuronal and mixed cultures, indicating a different mechanism of action for the two molecules. The specificity of the IL-1 beta effect was further demonstrated by the lack of response to treatment with other cytokines, including interleukin 2, interleukin 6, and tumor necrosis factor alpha. The cell type necessary for the IL-1 beta activity is probably the ganglion Schwann cell. Treatment with a synthetic immunosuppressant glucocorticoid, dexamethasone, blocked the increase in SP after treatment with IL-1 beta. These observations support the hypothesis that neuropeptide expression is regulated, in part, by interactions with specific immunoregulators. In addition, the data suggest a role for SP in mediating the response of the superior cervical ganglion to injury of the ganglion itself or to the fibers innervating it.