The O2 deficit-O2 debt relationship in ponies was examined at two levels of treadmill exercise. Five healthy ponies exercised for 8 min at two work rates (WR):(1) 50 m/min at 6% grade and (2) 70 m/min at 12% grade. The O2 deficit and O2 debt were calculated using open-circuit measurement of pulmonary gas exchange during the transition from rest to steady-state exercise and during recovery from exercise. The O2 deficit was significantly lower (P less than 0.05) than the O2 debt at each of the two work rates. The means (+/- SEM) for the deficit and the O2 debt, respectively, in liters were: (1) low WR = 0.45 +/- 0.12; 0.92 +/- 0.13; high WR = 1.03 +/- 0.20; 1.62 +/- 0.12. Given the observation that the O2 deficit and the O2 debt are generally equal in humans during exercise below the anaerobic threshold, these findings suggest a species difference in the O2 deficit-O2 debt relationship. It is hypothesized that the difference in O2 deficit between man and pony is related to faster VO2 kinetics for the on-transient of exercise which involves a more rapid increase in cardiac output coupled with a release of splenic stored erythrocytes.