Effects of hypoxia on the immune function of neonatal rats at the age of 14 days as well as on the levels of ACh, catecholamine in spleen were studied. After the animals were exposed to hypoxia at 5 km simulated altitude in hypobaric chamber for 5 days, there was 43.4% decrease in DNA content in spleen mono nuclear cell and a 13.2% decrease in mono nuclear cell proliferation. Similar suppression of these two parameters of immune function in exposure to 7 km for 24 h was also noted, which decreased by 39% and 19.8% respectively. The suppressive effect of 7 km for 24 h hypoxia on DNA content was partly blocked when rats were pretreated with DSP-4 intracerebroventricularly one day before hypoxia. The levels of catecholamine in spleen increased, while the levels of ACh decreased after 7 km exposure for 24 h. These observations indicate that hypoxia may suppress cellular function of neonatal rats and its action may be mediated by activation of sympathetic nervous system and inhibition of parasymphathetic one.