A free-paced 14.8-m stairclimb was compared to cycle ergometry at equivalent power outputs in six normal subjects. Heart rate (HR), inspiratory flow (VI) andd oxygen saturation in arterial blood (Sao2) were analyzed continuously, carbon dioxide output (Vco2) and oxygen consumption (Vo2) for the duration of exercise, and rebreathing mixed venous carbon dioxide pressure (Pvco2), capillary blood gases, and lactates at the completion of exercise. The average power output was 1,394 kpm/min sustained for 54.8 s. The HR and Vo2 responses were not significantly different between the two forms of exercise. VI was 20.3% less stairclimbing (P smaller than 0.005). A fall in Sao2 of 31.1% +/- 2.7) occurred in stairclimbing compared to 1.2% (+/- 2.1) in cycle ergometry. Differences in Sao2 between stairclimbing and cycling correlated with differences in VI (r = 0.80); falls in Sao2 could be prevented by overbreathing during stairclimbing or induced by controlled underbreathing during cycling. Vco2 and postexercise lactate levels were less stairclimbing (P smaller than 0.005 and P smaller than 0.01), as were Pvco2 measurements. The findings are consistent with a lower CO2 flow to the lung during stairclimbing than in cycling, which results in a lower alveolar ventilation and a consequent fall in Sao2.