Recent studies underline the importance of the immunoinflammatory processes in the pathology of Mg deficiency. Neutrophils possess a superoxide anion-generating NADPH oxidase and its inappropriate activation may result in tissue damage. The aim of the present study was to assess the effect of experimental Mg deficiency in the rat on polymorphonuclear leucocytes (PMN) activity and the role of increasing extracellular Mg. Weaning male Wistar rats were fed either a Mg-deficient or a control diet for 8 d. In Mg-deficient rats, the characteristic inflammatory response was accompanied by a marked increase in the number of PMN. Higher plasma interleukin 6 and NO concentrations and increased lipid peroxidation in the heart were found in Mg-deficient rats as compared with control rats. As shown by chemiluminescence studies, basal neutrophil activity from Mg-deficient rats was significantly elevated when compared with neutrophils from control rats. Moreover, the chemiluminescence of PMN from Mg-deficient rats was significantly higher than that of control rats following phorbol myristate acetate or opsonized zymosan activation. PMN from Mg-deficient rats also showed an increased activity of phagocytosis in comparison with neutrophils from control animals. Increasing extracellular Mg concentration in the incubating medium of PMN (0.8 v. 8.0 mM) decreased the chemiluminescence activity of PMN from control rats following opsonized zymosan activation. Chemiluminescence activities of PMN from Mg-deficient rats following phorbol myristate acetate or opsonized zymosan challenge were also decreased by high extracellular Mg concentration. From this work, it appears that PMN activation is an early consequence of Mg deficiency and that high extracellular Mg concentration inhibits free radicals generation.