Is hypercapnia necessary for the ventilatory response to exercise in man?

1. Continuous recordings of arterial pH, ventilation, airway CO2 and heart rate were made during rest and during 3-4 min periods of rhythmic leg exercise in four renal patients with arteriovenous shunts. 2. The patients were anaemic (haemoglobin 6.5-9.0 g/dl) but had a normal ventilatory response to exercise as judged by the ratio of the change in ventilation to the change in CO2 production. 3. Breath-by-breath oscillations in arterial pH disappeared for the majority of the exercise period in each patient. 4. Changes in mean arterial pH and end-tidal CO2 tension with exercise were inconsistent between subjects but consistent within a given subject. On average, mean arterial pH rose by 0.011 pH unit. Changes in end-tidal CO2 tension reflected changes in mean pHa by falling on average by 1 mmHg (0.13 kPa). 5. Hypercapnia and acidaemia were not found to be necessary for the ventilatory response to moderate exercise.