Objective. To study the effect of acute moderate hypoxia on plasma lactic acid (LA) and lactic dehydrogenase (LDH) content in pilots. Method. Concentrations of plasma LA and LDH were measured by colorimetry in 16 healthy male pilots in the early morning (6:00 a.m.), following acute moderate hypoxic exposure (10:00 a.m.) and 6 hours after returned to the ground (4:00 p.m.). 16 healthy male ground people served as control group, and their plasma LA and LDH contents were assayed at corresponding time. Result. Plasma LA after moderate hypoxic exposure in the pilot group was significantly higher than that at 6:00 a.m., 4:00 p.m., and control group (P < 0.01). While the content at 6:00 a.m., 4:00 p.m. in pilot group was not significantly different from the control group (P > 0.05). The content of plasma LDH after moderate hypoxic exposure in the pilot group was not significantly higher than that at 6:00 a.m., 4:00 p.m., and control group (P > 0.05). Conclusion. Acute moderate hypoxia can make the plasma LA content increase in pilots, and reach to the normal level within 6 h. While the LDH content has no significantly change.