1. The effects of 2,4-dinitrophenol (DNP) and sodium azide on the respiration of sea-urchin spermatozoa (Pseudocentrotus depressus and Hemicentrotus pulcherrimus) have been investigated in relation to dilution.
2. The initial outburst of respiration following dilution is higher in its rate and longer in its duration the more dilute the sperm suspensions are. After the initial high respiration has diminished, oxygen uptake in dense suspensions continues at a very constant rate for a long period of time, during which spermatozoa remain motile. A high level of respiration can be restored by further diluting a suspension in the steady state. In dilute suspensions, however, once the respiration has dropped, it can never be reactivated.
3. DNP (1 x 10-4M) causes a marked rise in oxygen uptake when given during the steady state, but has little effect during initial burst.
4. Conversely, only the initial high respiration is inhibited by sodium azide, while the subsequent steady respiration is not affected by this poison. This is especially the case in dense suspensions.
5. When the initial high respiration is inhibited by azide at 1 x 10-2M the reduced level always coincides with that of the steady-state respiration. The respiration of sea-urchin spermatozoa thus seems to be separable into two components, one azide-sensitive and the other azide-insensitive.
6. The extra increase in oxygen uptake over the steady-state level on adding DNP or Zn in dense suspensions is also ascribed to an increment in the azide sensitive component.
7. Potassium cyanide inhibits not only the initial increase but also the steady-state respiration.