1. The effects of treadmill exercise on oxygen consumption (V02), carbon dioxide production (VCO2), arterial blood lactate concentration ([L-]a), arterial blood pH and arterial gas tensions (PaO2 and PaCO2) were measured in 3 species of lizards (Varanus salvator, V. exanthematicus, Iguana iguana) 2. Varanus salvator was exercised 45 min at an intensity which required 85% of its VO2 max. V. salvator utilized supplementary anaerobic metabolism during the first 10 min of this sustainable exercise, as evidenced by a 16 mmol/l increase in [L-]a. Respiratory exchange ratios (R, where R = VCO2/VO2) exceeded 1.2 when [L-]a and [H+]a were maximal. One half of the accumulated lactate was removed from the blood during the remainder of the 45 min exercise period, while blood pH returned to resting levels. 3. In a second set of experiments, high intensity exercise led to exhaustion after 5 to 10 min in all three species, resulting in large lactate (+ delta[L-]a = 14-20 mmol/l) and hydrogen ion (+ delta[H+]a = 23-57 nmol/l) accumulations. R values ranged from 1.2-1.8 at exhaustion. 4. Recovery from both sustainable and non-sustainable exercise was characterized as a period of rapid lactate removal. Respiratory exchange ratios were low (0.3-0.5) as metabolic CO2 was retained, replacing depleted bicarbonate stores. 5. We conclude that all three lizard species make ventilatory adjustments during and after exercise that minimize disturbances to resting hydrogen ion concentrations and acid-base balance. Varanus salvator demonstrate the ability to re-establish resting acid-base status during sustained exercise requiring 85% of their VO2,max. Changes in R appear to be a useful noninvasive indicator of net blood lactate accumulation.