Historically, the sympathetic nervous system (SNS) has been mostly associated with the 'fight or flight' response and the regulation of cardiovascular function. However, evidence over the past 30 years suggests that SNS may also influence the function of immune cells. In this review we describe the basic research being done in the area of SNS regulation of immune function. Further, we show that the SNS-immune interplay during circadian rhythm may modulate the robustness of the inflammatory response, critical for survival during periods of increased activity. Finally, new concepts of a close relationship between these systems in the pathogenesis of hypertension are discussed.