Chemiluminescence analysis was used to examine the yeast cell-activation of rat whole blood neutrophilic leukocytes in rats after their pre-exposure to 3-hour hypoxia (9,000 m above sea level) and two-hour hyperoxia (0.3 MPa), as well as their resultant effects. The hypoxia was found to cause a significant (42%) reduction in the length of the latent period, suggesting that it activates the phagocytic activity of neutrophilic leukocytes. At the same time hyperoxia leads to a delayed response of competent cells to yeast cell stimulation. On subsequent exposure to hypoxia and hyperoxia, drastic inhibition of drastic cellular activation occurred, which manifested itself by a 62% increase in the length of the latent period and by a 38% rise in the appearance time of luminescence peak. Thus, the findings suggest that it is essential to bear in mind the fact that phagocytosis can decrease if oxygen is overdosed during hyperbaric oxygenation and in hypoxic states.