IN the natural sequence of mammalian fertilization, the spermatozoa await the arrival of ova in the Fallopian tubes. The time of waiting may vary from 6 hr. (rabbit), 24 hr. (ferret), 20–30 hr. (pig, sheep and cattle) up to perhaps several months (bat)¹. Since the transportation of sperm from the vagina to the upper part of the tube is very rapid (a few minutes to 6 hr.¹), and since the number of spermatozoa present at the site of fertilization is very small¹, this time interval is not necessarily for the accumulation of a large number of spermatozoa to ensure fertilization. The following experiment demonstrates that such a period of time in the female tract is required for the spermatozoa to acquire their fertilizing capacity.