The present study investigated the respiratory rates, frequency of air breathing and ability to extract oxygen from air compared to body mass of three neotropical air-breathing fishes (Lepidosiren paradoxa, Hoplerythrinus unitaeniatus and Hoplosternum littorale) with different accessory organs, under the same conditions of hypoxia (PO(2)<5 mm Hg) and temperature (25 degrees C), using the same apparatus. The results indicated a superior capacity of H. littorale (16.01 ml O(2) breath(-1) kg(-1)) compared to H. unitaeniatus (7.03 ml O(2) breath(-1) kg(-1)) and L. paradoxa (4.95 ml O(2) breath(-1) kg(-1)).