The staphylococcal enterotoxin B (SEB)-induced immediate-type skin reaction in unsensitized monkeys was used as a nonimmunologic mast cell stimulation to search for possible involvement of local neural mechanisms. Evidence is presented that substance P (SP) plays a predominant role in mediating intradermal SEB challenge in unsensitized monkeys. With a rabbit SP antiserum directed against the C-terminal region of SP, a concentration-dependent inhibition of SEB-induced skin reactivity could be demonstrated. Furthermore, a rabbit antiserum directed against the mast cell activating N-terminal part of SP was capable of impeding SEB-induced skin reactions totally. By use of SP antagonists, significant reduction of skin reactions evoked by SEB was found. Finally, capsaicin pretreatment of the skin caused a substantial inhibition of SEB-induced skin reactivity. These data suggest that SEB exerts its effect on cutaneous mast cells via stimulation of primary sensory neurons that contain SP. Moreover, a new in vivo model is described for studies of nerve-mast cell interactions.