Abstract Pacific tarpon ( Megalops cyprinoides) use a modified gas bladder as an air-breathing organ (ABO). We examined changes in cardiac output ( V̇ b) associated with increases in air-breathing that accompany exercise and aquatic hypoxia. Juvenile (0.49 kg) and adult (1.21 kg) tarpon were allowed to recover in a swim flume at 27 °C after being instrumented with a Doppler flow probe around the ventral aorta to monitor V̇ b and with a fibre-optic oxygen sensor in the ABO to monitor air-breathing frequency. Under normoxic conditions and in both juveniles and adults, routine air-breathing frequency was 0.03 breaths min − 1 and V̇ b was about 15 mL min − 1 kg − 1 . Normoxic exercise (swimming at about 1.1 body lengths s − 1 ) increased air-breathing frequency by 8-fold in both groups (reaching 0.23 breaths min − 1 ) and increased V̇ b by 3-fold for juveniles and 2-fold for adults. Hypoxic exposure (2 kPa O 2) at rest increased air-breathing frequency 19-fold (to around 0.53 breaths min − 1 ) in both groups, and while V̇ b again increased 3-fold in resting juvenile fish, V̇ b was unchanged in resting adult fish. Exercise in hypoxia increased air-breathing frequency 35-fold (to 0.95 breaths min − 1 ) in comparison with resting normoxic fish. While juvenile fish increased V̇ b nearly 2-fold with exercise in hypoxia, adult fish maintained the same V̇ b irrespective of exercise state and became agitated in comparison. These results imply that air-breathing during exercise and hypoxia can benefit oxygen delivery, but to differing degrees in juvenile and adult tarpon. We discuss this difference in the context of myocardial oxygen supply.