1. The O2 uptake of Echinus esculentus sperm suspensions is strongly inhibited by CO. In equilibrium with a mixture of 90% CO and 10% O2, the O2 uptake is 60% less than that of an identical suspension in equilibrium with 90% N2 and 10% O2.

2. This inhibition is completely reversed by light.

3. Spectroscopic examination revealed cytochromes a, a3, b and c. On treatment with CO the spectrum of COa3 was observed.

4. These results establish that the cytochrome system is the catalytic agent which controls the respiration of sea-urchin spermatozoa.

5. The O2 uptake of sperm suspensions in equilibrium with 90% N2 and 10% O2 is about 10% lower than in air.

6. Certain difficulties in measuring the R.Q. of sperm are discussed.

7. The reversible effect of CO on the O2 uptake of spermatozoa is the same whether they are in egg water or sea water.

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