1. The effects of O2, CO2, and pressure were studied in two very different species of protozoa, a flagellate, Chilomonas paramecium, grown in acetate-ammonium solution and a ciliate, Tetrahymena geleii, grown in 2 per cent proteose-peptone solution. 2. Chilomonas and Tetrahymena live and reproduce in solutions exposed to a wide range of O2 concentrations, but Chilomonas is killed at high O2 tensions in which Tetrahymena grows best. The optimum O2 concentration for Chilomonas is about 75 mm. pressure but it lives and reproduces in O2 tensions as low as 0.5 mm. while Tetrahymena fails to grow in concentrations below 10 mm. O2 pressure. 3. With a constant O2 tension of 50 mm. pressure, it was found that there is no significant variation in growth in Chilomonas between 50 mm. and 740 mm. total pressure. In Tetrahymena, however, under the same conditions, an optimum total pressure was found at about 500 mm. and growth is comparatively poor at 50 mm. total pressure. 4. Tetrahymena does not live very long in CO2 tensions over 122 mm., although Chilomonas grows as well at 400 mm. CO2 as in air at atmospheric pressure (0.2 mm. CO2). Tetrahymena grows best in an environment minus CO2, but the optimum for Chilomonas is 100 mm. CO2 at which pressure an average of 668,600 ± 30,000 organisms per ml. was produced (temperature, 25 ± 1° C.). 5. Chilomonads grown in high CO2 concentrations (e.g., 122 mm.) produce larger starch granules and more starch than those grown in ordinary air at atmospheric pressure. 6. In solutions exposed to 75 mm. O2 tension (optimum) and 122 mm. CO2 plus 540 mm. N2 pressure, chilomonads contain very little, if any, fat. This phenomenon seems to be due to the action of CO2 on the mechanisms concerned with fat production. 7. In Tetrahymena exposed to pure O2, there is very little fat compared to those grown in atmospheric air. This may be due to the greater oxidation of fat in the higher O2 concentrations. 8. Further evidence is presented in support of the contention that Chilomonas utilizes CO2 in the production of starch.

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Unicellular organisms make excellent material for the study of various cellular phenomena. Tctrah\nncna t/clcii, a colorless holotrichous ciliate, is an exceptionally desirable organism for such physiological studies, mainly because it is readily grown in rather simple, sterile organic media. This organism contains cytochromes c, b, a, and possibly a._, and its oxygen consumption is inhibited by cyanide and carbon monoxide (Baker and Baumberger, 1941). It grows well at ordinary oxygen tensions, but is most prolific in pure "oxygen (Pace and Ireland, 1945). It soon dies in oxygen tensions below 10 mm. Hg partial pressure.Many investigations have been carried out with Tetrahymena as the experimental organism. There is, however, much to be desired in respect to our knowledge concerning respiratory metabolism ; knowledge that may lead to a better understanding of the respiratory mechanisms of cells in general. Since very little is known concerning oxygen consumption, carbon dioxide elimination, and respiratory quotient in Tetrahymena, the following investigations were made.