This study investigated whether in trained middle-distance runners, intermittent short-term graded running performance is affected by a hypobaric hypoxic environment (simulated 2,500 m) (H). Seven male middle-distance runners performed an aerobic performance test and an intermittent short-term graded anaerobic running-performance test (MART) both in H and in a normobaric normoxic environment (N). VO(2max) and OBLA were markedly lower (by 18.1% and 8.7%, respectively) in H than in N. In MART, neither maximal running velocity (V(max)) nor exhaustion-time was different between N and H (454 (7) m min(-1) vs. 451 (6) m min(-1), respectively, and 208.7 (5.2) s vs. 205.7 (4.2) s, respectively). The blood lactate concentration at sub-maximal running speed (425 m min(-1)) was significantly greater in H than in N (paired t-test: P<0.05). These results suggest that, in trained middle-distance runners, intermittent short-term graded running performance is not affected by H, despite a considerable decrease in aerobic power in H during the aerobic performance test.