We have recently shown that substance P (SP) participates in the stress-induced modulation of elicited, peritoneal macrophage function. This study reports the in vitro effects of SP on macrophage activity. We show by an MTT bioassay that SP significantly increases cellular metabolic activity. We show by ELISA that preincubating (priming) the macrophages with SP, prior to the incubation with lipopolysaccharide (LPS), results in a significant enhancement of proinflammatory cytokine secretion, relative to LPS alone. Finally, we show that somatostatin can antagonize the SP-induced enhancement of cytokine secretion. The above results demonstrate the importance of the temporal sequence in which stimuli are administered, in vitro, and indicate that SP can act as a first signal in the cascade of macrophage activation. We postulate that stress, via the secretion of SP and other sensory neuropeptides, may play a role in the pathogenesis of certain inflammatory diseases of unknown etiology.