1. Glutamine is an amino acid that is used to treat various diseases. Glutamine has been reported to have protective effects in human pulmonary epithelia-like cells exposed to hyperoxia. However, the effects of glutamine in hyperoxia-induced lung injury have not been investigated in vivo. 2. Mice treated with saline or glutamine [(750 mg/kg) intravenously] were randomly exposed to hyperoxia for 48 or 72 h. Control mice treated with saline or glutamine were exposed to room air. Cytokine levels in bronchoalveolar lavage fluid (BALF), heat shock protein (HSP) 70, the wet/dry (W/D) weight ratio, malondialdehyde (MDA) levels, myeloperoxidase (MPO) activity and pathoglogical findings in lung tissue were evaluated to determine the effects of glutamine on acute lung injury. In addition, survival was monitored. 3. Lung expression of HSP70 was significantly enhanced in both the control (room air) and 48 and 72 h hyperoxic glutamine-treated mice. The W/D ratio, BALF concentrations of tumour necrosis factor-alpha and interleukin-6, MDA levels, MPO activity, neutrophil infiltration and interstitial oedema in lung tissue were significantly lower at 48 and 72 h of hyperoxia in glutamine-treated mice compared with saline-treated mice. 4. In a separate series of experiments evaluating survival, after 96 h continuous exposure to hyperoxia, all saline-treated mice died. In contrast, all glutamine-treated mice died after 108 h exposure to hyperoxia. 5. The data suggest that glutamine administered to mice during hyperoxia has a protective effect against hyperoxia-induced acute lung injury and improves survival.