1. The oxygen consumption of suspensions of rabbit sperm obtained from the vas deferens or the large tubules of the cauda epididymis is maintained at its initial value for a maximum time when the pH of the medium is 7.8-8.0.
2. When the pH of the medium is on either the acid or alkaline side of this value, and within the range pH 6.4-9.0 the active life of the suspension is less but the initial consumption is unaltered.
3. In the majority of the experiments the presence of thyroxine had no effect upon the oxygen consumption at pH 7.8-8.0, but it prolonged the active life of the suspension when the pH of the medium was on either the acid or alkaline side of this value.
4. Des-iodo-thyroxine did not produce this effect.
5. These results are compared with those of previous experiments on the sperm of echinoderms. It is concluded that in the ripening of mammalian, as of echinoderm, sperm a substance related to thyroxine is elaborated within the sperm.