Background: Underwater rugby is a team sport where players try to score points with a negatively buoyant ball while submerged in a swimming pool. Reports of syncope incidents at the Swedish Championships led to us to investigate end-tidal oxygen and carbon dioxide levels during simulated match play. Methods: Eight male underwater rugby club players of varying experience participated. Repetitive measurements were made while players were defending during simulated match play. Each time a player surfaced they exhaled through a mouthpiece connected to a flow meter and a gas analyzer to measure tidal volume, PETO2 and PETCO2. Results: Measurements were made over 12 dives, with an average dive duration of 18.5 seconds. The mean maximal PETCO2 across the eight participants was 10.0 kPa (similar to 75 mmHg) (range, 9.1-11.7 [-68-88]). The corresponding mean minimum PETO2 was 7.6 kPa (similar to 57 mmHg) (6.3-10.4 [similar to 47-78)). PETCO2 drifted upward, with the mean upward change from the first to last dive for each participant being +1.8 (similar to 13.5 mmHg) (SD 1.74) kPa. A similar trend for PETO2 was not detected, with a mean change of similar to 0.1 (similar to 0.75 mmHg) (SD 3.79) kPa. Conclusion: Despite high PETCO2 values that were close to narcotic being recorded, these players seemed to regulate their urge to breathe based on hypoxia rather than hypercapnia.