1. Factors controlling aerial and aquatic breathing have been studied in the intact, free-swimming African lungfish, Protopterus aethiopicus. Frequencies of aerial and branchial breathing were correlated with gas tensions in the lung and in blood from pulmonary and systemic arteries and veins.

2. Studies were made, on fish resting in aerated water, during exposure to air and during hypoxic and hypercarbic conditions in the environment.

3. Both branchial and pulmonary breathing were irregular during rest in aerated water. The rate of branchial breathing normally exceeded that of air breathing. An increased rate and vigour of branchial breathing commonly preceded an air breath. There was no indication that a new air breath was related to the values of arterial Po2 and pH.

4. Air exposure elicited a marked and immediate increase in the rate of air breathing. Hypoxic water never evoked any compensatory breathing responses while breathing from a hypoxic gas mixture quickly increased the rate of air breathing. Bubbling 5% CO2 in air into the aquarium caused a reduction in branchial breathing while the rate of air breathing increased.

5. Nicotine injected intravenously or in the water close to the gills elicited an increase in both aerial and branchial breathing.

6. Respiratory and circulatory events were correlated during undisturbed breathing and during artificial lung inflation. Increased cardiac output and a shift in regional blood flow to a higher pulmonary flow occurred with air breaths.

This work was supported by grant GB 4038 from the National Science Foundation and grant HE-08405 from the National Institute of Health.
 Established Investigator of the American Heart Association. Supported by the Northeastern Chapter, Washington State Heart Association.