The purpose of this study was to determine the effect of heavy thermal panting on arterial oxygen (PaO2) and carbon dioxide (PaCO2) tension in emus. The birds showed no significant change in body temperature during a 3-4 h heat stress caused by increasing ambient air temperature from 21 to 46 degrees C. However, the emus increased their respiratory frequency 10-fold (from 5.3 to 52.9 breaths X min-1). The high respiratory frequency resulted in a slight but significant decrease in PaCO2 (from 33.5 to 29.8 mm Hg), coupled with a slight increase in pH (from 7.449 to 7.469). Paradoxically, these changes were accompanied by a significant decrease in the arterial oxygen tension (from 99.7 to 84.6 mm Hg). The arterial hypoxia suggests hypoventilation while the hypocapnia suggests hyperventilation of the lungs. This could result from various spatial and/or temporal changes in ventilation/perfusion ratios.