Extrapolation from post-exercise measurements has been used to estimate respiratory and blood gas parameters during exercise. This may not be accurate in exercise with reduced breathing frequency (RBF), since spontaneous breathing usually follows exercise. This study was performed to ascertain whether measurement of oxygen saturation and blood gases immediately after exercise accurately reflected their values during exercise with RBF. Eight healthy male subjects performed an incremental cycling test with RBF at 10 breaths per minute. A constant load test with RBF (B10) was then performed to exhaustion at the peak power output obtained during the incremental test. Finally, the subjects repeated the constant load test with spontaneous breathing (SB) using the same protocol as B10. Pulmonary ventilation (V_E), end-tidal oxygen (P_ETO₂), and carbon dioxide pressures (P_ETCO₂) and oxygen saturation (SaO₂) were measured during both constant load tests. The partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) in capillary blood were measured during the last minute of exercise, immediately following exercise and during the third minute of recovery. At the end of exercise RBF resulted in lower P_ETO₂, SaO₂ and PO₂, and higher P_ETCO₂ and PCO₂ when compared to spontaneous breathing during exercise. Lower SaO₂ and P_ETO₂ were detected only for the first 16s and 20s of recovery after B10 compared to the corresponding period in SB. There were no significant differences in PO₂ between SB and B10 measured immediately after the exercise. During recovery from exercise, P_ETCO₂ remained elevated for the first 120s in the B10 trial. There were also significant differences between SB and B10 in PCO₂ immediately after exercise. We conclude that RBF during high intensity exercise results in hypoxia; however, due to post-exercise hyperpnoea, measurements of blood gas parameters taken 15s after cessation of exercise did not reflect the changes in PO₂ and SaO₂ seen during exercise.