Varanids in general exhibit greater aerobic capacities than other lizards. In a similar approach to the extensive investigations undertaken in mammals, the respiratory system in varanids is examined in terms of oxygen transfer from the air to the blood during rest and sustained locomotory activity. The parameters controlling the transfer of O(2) through the various steps of the respiratory system are appropriate to meet the maximum demands for oxygen with one possible exception, circulatory convection. Ventilatory convection is maintained during maximal aerobic locomotion ensuring adequate pulmonary ventilation and the protection of alveolar P(O(2)). Little evidence exists to indicate a mechanically imposed constraint to breathe and the possibility of a gular pump acting to assist ventilation, as a general feature of varanids remains to be determined. Alterations in the relative contributions of the ventilation-perfusion ratio, pulmonary diffusion, diffusion equilibrium and right-left shunts preserved the alveolar-arterial P(O(2)) difference, ensuring that arterial oxygenation was maintained. In those species where increases in cardiac output were limited, maximum O(2) transfer was achieved through increased extraction of oxygen at the tissues. Overall, the interrelationship of adjacent steps in the respiratory system ensures that one step cannot become limiting. Compensatory changes occur in various parameters to offset those parameters that are 'limited'. The high aerobic activity of varanid lizards would not be achievable without a compensated circulatory convection.