1. Respiratory properties of blood and pattern of branchial and pulmonary gas exchange have been studied in twelve specimens of the South American lungfish, Lepidosiren paradoxa (Fitz).

2. Haematocrit ranged from 14 to 19% and blood oxygen capacity from 4.9 to 6.8 vol. %. The blood had a high affinity for O2 with a P50 value of 10.5 mm. Hg at Pco2 6 mm. Hg and temperature 23° C. The Bohr effect was low.

3. The CO2 dissociation curves show a steep ascending slope resulting in a relatively high CO2 combining power at physiological values of blood Pco2 The Haldane effect was small. Buffering capacity of oxygenated whole blood was high and exceeded that in typical water breathers.

4. Air breathing was prominent and intervals between air breaths varied from 3 to 10 min. Branchial respiratory movements were extremely shallow and showed a labile frequency. Air breathing was stimulated by hypoxic and hypercarbic water while hyperoxygenated water had no effect. Branchial respiratory rate showed a marked acceleration in response to mechanical agitation of the water.

5. Gas exchange was predominantly carried out by pulmonary breathing. In less than 10 min. the PO2 of expired gas dropped from 150 mm. Hg to less than 30 mm. Hg. The shallow branchial breathing with very low ventilation values resulted in a low O2 uptake via the gills.

6. Blood-gas analysis documented a clear selective passage of blood through the only partially divided heart. A consistently higher PO2 in dorsal aortic than in pulmonary arterial blood indicates a preferential passage of pulmonary venous blood to the anterior branchial arteries giving rise to most of the systemic circulation while systemic venous blood was largely conveyed to the most posterior branchial arteries giving rise to the pulmonary arteries.

7. The oxygen uptake for fish resting in water with access to air averaged 53.4 ml./hr./kg. Exposure to air lowered the O2 uptake markedly.

8. The increased importance of pulmonary breathing in Lepidosiren is discussed in relation to the transition from water breathing to air breathing.

This investigation was supported by grant G.B. 358 from the National Science Foundation and grants H.E. 0845 and H.E. 01892 from the National Institute of Health.
 This work was carried out during the tenure of an Established Investigatorship of the American Heart Association.