25 mongrel dogs (average b.w. 24.6 kg) were studied on several occasions at rest and during treadmill exercise of up to 10 mph (15% incline). Minute ventilation (VE), oxygen consumption (VO2), carbon dioxide production (VCO2), tidal volume (VT) and respiratory frequency (f) were determined at rest and at each level of exercise. Individual variability in resting VO2 was considerable (71--695 ml/min). Most often the dogs panted, with VE's above 25 liters/min and f's above 100 min-1. The averate VE/VO2 was 109 at rest. VO2 was linearly related to VE (VO2 = 9.17 VE + 66.9; r = 0.80). Differences in resting VE were largely due to differences in f (f = 3.57 VE + 21.2; r = 0.82). Considerable individual variability in VO2 for a given work load was also observed during exercise. Some dogs showed significant differences in VO2 from experiment to experiment while running at a given treadmill speed. These differences were largely related to the levels of VE. VE/VO2 decreased to 50. We found a leveling off of VO2 (at about 60 ml/min/kg) at treadmill speeds of 5 mph, suggesting that the maximal VO2 in dogs is less than previously reported.