ABSTRACT In a previous paper (Carter, 1930) it was shown that the presence of thyroxine produces effects upon the oxygen consumption of suspensions of the spermatozoa of Echinus miliaris which are in some respects similar to those produced by the secretions of the eggs of the same species. It was suggested that the secretions contain a body which has similar physiological action to thyroxine, and may be related to it in chemical structure. The observations to be recorded in the present paper form a part of an investigation undertaken with the object of comparing the action of these substances on the phenomena of fertilisation more widely. In this paper the results of further experiments on the oxygen consumption of suspensions of spermatozoa are given. Other aspects of fertilisation will be considered in later papers. Although it is clear that such an investigation should not be confined to the gametes of one species or of one group of animals, it is also clear that the study of each species should be as complete as possible. For this reason the present paper is confined to two species of Echinus, E. esculentus and E. miliaris, the spermatozoa of which behave very differently. The experiments discussed in the previous paper were concerned with the second of these two species, and the results on that species here given are intended to extend those given in that paper.