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.