1. Oxygen and carbon dioxide tensions were determined in the lungs and in blood from the dorsal aorta, pulmonary vein, pulmonary artery and inferior vena cava in the intact, free swimming, Amphiuma. At 15° C this animal was submerged for a large part of the time and surfaced briefly to breathe at variable time intervals, the mean period being 45 min.

2. Oxygen tensions in the lungs and in all blood vessels oscillated with the breathing cycles, falling gradually during the period of submersion and rising rapidly after the animal breathed. The absolute level of oxygen tension did not appear to constitute the effective signal beginning or ending a series of breathing movements.

3. A small oxygen gradient existed between lungs and blood in the pulmonary vein immediately after a breath. The gradient increased in size as an animal remained submerged due, it is suggested, to lung vasoconstriction increasing the transfer factor.

4. Blood in the dorsal aorta had a lower oxygen tension than that in the pulmonary vein. A right-to-left shunt occurred as blood moved through the heart. The degree of shunting increased as the animal remained submerged and pulmonary vasoconstriction occurred. Left-to-right shunt was relatively insignificant since oxygen tensions in the inferior vena cava and pulmonary artery were very similar.

5. Carbon dioxide tensions were relatively constant during the breathing-diving cycle since Amphiuma removed almost all of this gas through the skin.

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