['To investigate the effects of inhalation of hydrogen gas on cognitive impairment induced by transient cerebral ischemia and its potential mechanism.', 'Two-vessel occlusion rat model was used to produce 10-minute transient global  cerebral ischemia. One hundred and twenty male Wistar rats were randomly divided into sham, sham+H(2), ischemia, and ischemia+H(2) groups (n = 30 each group). Inhalation of 2% hydrogen gas was performed immediately at the end of operation and lasted for 3  hours. Cognitive function of rats was evaluated via Morris water maze. Neuronal damage in the CA1 region was quantified according to their morphological changes revealed by hematoxylin-eosin staining. The levels of oxidative stress products malondialdehyde (MDA) and 8-iso-prostaglandin  F2alpha, and the activities of anti-oxidative enzymes catalase and superoxide dismutase were measured to investigate the effects of inhalation of hydrogen gas on oxidative stress.', 'Inhalation of hydrogen gas decreased significantly the average latency of the ischemic rats  in finding hidden platform and elongated markedly their retention in the target quadrant. The neuronal density 3·3±2·1 cells/mm in CA1 region of the ischemic rats increased to 21·7±2·6 cells/mm after they were treated with hydrogen gas. Moreover, hydrogen  gas made higher levels of MDA and 8-iso-PGF2α in the ischemic rats attenuate to 3·2±0·2, 3·5±0·5, 3·4±0·3 and 26·4±2·3, 28·2±2·6, 26·8±2·1 at reperfusion  4, 24, and 72 hours, respectively (P<0·01 versus ischemia group at each indicated time). By contrast, the activities of superoxide dismutase and catalase damaged by ischemia/reperfusion recovered to 129·7±14·8, 100·5±12·2 and 11·4±0·8,  9·6±1·1 at reperfusion 24 and 72 hours, respectively (P<0·01 versus ischemia group at each indicated time).'] Does inhalation of hydrogen gas attenuate cognitive impairment in transient cerebral ischemia via inhibition of oxidative stress?