Influence of magnesium pre-treatment on effects of repetitive hypoxia was studied in the hippocampus of rats. Rat pups were kept for 8 hours a day in a hypobaric chamber at a simulated altitude of 7 000 m since the day of birth till the 17^th day. The first group of animals was exposed to the repeated hypoxia, the second group under the same conditions was pre-treated by magnesium. Both groups were compared with intact control animals and intact animals treated with magnesium. Brains of 35-day-old experimental and control animals were processed to visualize nitrergic neurons (NADPH-d staining). Intermittent hypoxia brings about higher numbers of NADPH-d positive neurons in CA1 and CA3 areas of the hippocampus and in the dorsal blade of the dentate gyrus, in the comparison with either group of control animals. In the hilus and ventral blade of the dentate gyrus the number of NADPH-d positive neurons was lower. Magnesium pre-treatment during hypoxia decreased the number of nitrergic neurons in all areas of the hippocampus except the CA1 area. These results demonstrate that magnesium possible has a neuroprotective effect. This work was supported by grants GACR 305/03/H148, GACR 309/05/2015, GA UK 45/2004 and MSM 0021620816.