Nine young athletes performed treadmill exercise at average VO2 levels of 1.08, 1.78, 3.00, and 3.57 l/min while exposed to inspired PCO2 levels of 0, 10, 20, 30, and 40 Torr. The average slope of the VE-PACO2 relationship increased significantly from 3.59 to 4.70 l.min-1.Torr-1 in the transition from rest to light exercise and then decreased progressively and significantly to 1.34 l.min-1.Torr-1 at the highest work load. A similar pattern of initial increased steepness followed by progressive flattening was found in the VT-PACO2, curves, whereas slopes of the f-PACO2 relationships continuously decreased in the transition from rest through light to heavy exercise. These data are consistent both with an increased ventilatory response to PACO2--[H+]a increments during light exercise and with progressive reduction of that response during the transition from light to heavy exercise in parallel with increasingly severe mechanical influences on VE as maximal limits are approached. They are not consistent with simple addition of chemical and nonchemical components of exercise hyperpnea at all levels of exercise.