The regulatory role of substance-P (SP) on neutrophil and endothelium activation as well as nitric oxide (NO) production induced by Mg-deficiency was examined. Male Sprague-Dawley rats (180 g) were fed either a Mg-deficient (MgD) or Mg-sufficient (MgS) diet for 3 weeks. Enriched neutrophil fractions (> 85%) isolated from whole blood of the Mg-deficient rats displayed an 11-fold (p < 0.001) higher basal superoxide anion producing activity (assayed as SOD-inhibitable cytochrome c reduction) compared to that obtained from the MgS rats. Treatment of the MgD rats with the specific SP-receptor (SPR) blocker, L-703,606 (1 mg/kg/day as s.c. implanted sustained-release pellets) attenuated the superoxide anion producing activity by 75% (p < 0.025). In parallel, circulating prostacyclin (PGI2) level (assayed as 6-keto-PGF-1alpha) was elevated 13-fold in the MgD rats, but was reduced 90% by L-703,606 treatment. Concomitantly, plasma NO products (nitrate + nitrite), which increased 2.2-fold during Mg-deficiency, were completely suppressed by the SPR blockade. When the isolated hearts were subjected to ischemia/ reperfusion stress, NO products were elevated 2.4-fold in the effluent of the MgD group compared to MgS; such heightened NO release was also attenuated after in vivo treatment with the SPR blocker. In conclusion, SP plays a direct role in promoting activation of the neutrophil and endothelium as well as induction of NO production; these processes might participate in the oxidative stress that contributes to the depletion of blood glutathione and cardiac pathology.