The effect of glutamine on the activity of the NADPH oxidase complex from rat neutrophils was investigated. Superoxide anion (O 2 - ) production was assessed: (1) by scintillation counting by using lucigenin, and (2) by reduction of cytochrome c over 10min. The effects of glutamine and PMA on the expression of the NADPH oxidase components p22 phox , gp91 phox and p47 phox were also determined. Glutamine at 1 and 2mM increased O 2 - generation in the presence of PMA by 100% and 74% respectively, in neutrophils maintained previously for 3h in medium deprived of this amino acid. DON (6-diazo-5-oxo- l -norleucine), an inhibitor of phosphate-dependent glutaminase and thus of glutamine metabolism, caused a significant decrease in O 2 - production by neutrophils stimulated with PMA both in the absence (44%) and in the presence (66%) of glutamine. PMA markedly increased the expression of gp91 phox , p22 phox and p47 phox mRNAs. Glutamine (2mM) increased the expression of these three proteins both in the absence and in the presence of PMA. We postulate that glutamine leads to O 2 - production in neutrophils, probably via the generation of ATP and regulation of the expression of components of NADPH oxidase.