Previous measures of cardiovascular endurance after repeated dives on air and 100% oxygen at 1.35 atmospheres absolute (ATA) have shown reductions in performance that persist up to 3 days post-dive for the hyperoxic group only. PURPOSE: Examine ventilatory parameters associated with changes in treadmill endurance immediately and 3 days after completing repeated air and oxygen resting dives. METHODS: 15 and 12 healthy male Navy divers completed 5 consecutive 6-hr dives with 18-hr surface intervals while breathing air and 100% oxygen, respectively, at 1.35 ATA (Air/Oxygen: 30±5/32±7 yrs; VO 2max: 52±7/53±5 ml/kg/min; mean±SD). Treadmill endurance time at 80-85% of VO 2max and associated physiological variables were tested a few days prior to the first dive (BL), 2 hours post-dive day 5 (PD) and 3-days post-dive (PD3) on day 8. Breath-by-breath (Innocor) minute ventilation (VE), end-tidal CO2 (FETCO2), oxygen consumption (VO 2), and respiratory quotient (R) were collected during the 80-85% VO 2max run at 10% grade until exhaustion. PD and PD3 were compared to BL (BL-PD and BL-PD3) using data from the first minute, mid-run, and last minute of the run. RESULTS: PD cardiovascular endurance significantly decreased for Air and Oxygen phases (Air: -34%; Oxygen: -36%; p<0.05), yet only the Oxygen phase remained reduced PD3 (Air: -11.9%; p>0.05; Oxygen: -31%; p<0.05). VE increased significantly during the run, but was not different between Air and Oxygen phases or across testing days. Although FETCO2 decreased across run times and testing days for Air and Oxygen, values for the Oxygen group remained lower than Air throughout (p<0.05). PD VO 2 showed decline in both groups (Air/Oxygen: PD -7%; p<0.05). VO 2 for Air recovered by PD3, whereas Oxygen remained reduced (Air: +1%; Oxygen: -13%; p<0.05). PD R increased overall for both groups, yet Air returned to baseline while Oxygen remained elevated by PD3 (Air: PD: +4%; p<0.05; Oxygen: PD +7%, PD3 +4%; p<0.05). CONCLUSIONS: Treadmill endurance is reduced after long-duration diving and persists longer when exposed to hyperoxic diving conditions. Reduced VO 2 and elevated R has been reported in the literature to correlate with increased lactate production and a greater reliance on anaerobic energy systems making them likely causes of this decrease in aerobic performance.