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Purpose: Although caffeine is known to possess ergogenic effects, previous studies demonstrated no effect of caffeine on 800-m run performance outdoors, which might be due to several uncontrolled factors including pacing strategies. We hypothesized that caffeine ingestion improves a pace-controlled simulated 800-m run performance. We also hypothesized that exercise-induced arterial hypoxemia occurs during the simulated 800-m run, and this response is mitigated by caffeine-induced increases in exercise ventilation. Methods: In a randomized, double-blind, placebo-controlled and crossover design, 16 (3 females) college middle-distance runners who have 800-m seasonal best of 119.97 ± 7.64 s ingested either 1) placebo (6 mg of glucose per kilogram of body weight) or caffeine (6 mg of caffeine per kilogram of body weight). Then they performed an 800-m run consisting of 30-s running at 103% of their 800-m seasonal best, followed by running at 98% of seasonal best until exhaustion, which mimics actual 800-m run pacing pattern. Results: Running time to exhaustion was extended by 7.3% ± 6.2% in the caffeine-ingested relative to placebo trial (123 ± 12 vs 114 ± 9 s, P = 0.04). Arterial oxygen saturation markedly decreased during the simulating running, but this response was similar (76.6% ± 5.7% vs 81.1% ± 5.2%, at 113 s of the simulating running) between the caffeine and placebo trials (P ≥ 0.23 for time–supplement interaction and main effect of supplement). Minute ventilation, oxygen uptake (all P ≥ 0.36 for time–supplement interaction and main effect of supplement), and rate of perceived exertion (all P ≥ 0.11) did not differ between the trials throughout the simulating running. Heart rate was higher in the caffeine-ingested trial throughout the simulated running (P < 0.01 for main effect of supplement). Postexercise blood lactate concentration was higher in the caffeine trial (P = 0.02). Conclusions: Caffeine ingestion improves simulated 800-m run performance without affecting exercise ventilation and severe exercise-induced arterial hypoxemia. [ABSTRACT FROM AUTHOR]