Abstract 1. 1. Oxygen consumption, ventilation frequency and tidal volume were measured simultaneously in two lizards, Varanus gouldii and Sauromalus hispidus, during rest and maximal activity. 2. 2. Ventilation frequency and tidal volume are almost identical in these species. The greater oxygen consumption during activity of Varanus results from a greater oxygen extraction from ventilated air. 3. 3. Increased oxygen consumption during activity results from increments in tidal volume; ventilation frequency remains constant. 4. 4. The thermal and weight dependence of ventilatory variables in reptiles is analyzed. 5. 5. The capacity for oxygen extraction by saurian lungs is similar to that of mammals and birds, even at identical ventilation rates, in spite of their simple lung morphology.