SUMMARY: Using the techniques of sperm transfer into the oviduct of recipient females, or ovum transfer to the site of capacitation, it has been shown that the capacitating activity of the female genital tract may be modified by oestrogen and by progesterone. The normal capacitating potential of the Fallopian tube is diminished after ovariectomy, and is restored by oestrogen injection. In the absence of oestrogen (in the ovariectomized and adrenalectomized female) this activity is still maintained at a basal level sufficient to capacitate the numbers of sperm which normally enter the Fallopian tube after coitus. Capacitation is not significantly disturbed in the Fallopian tube of the intact progesterone-dominated female (Chang, 1958), and the innate basal capacitating activity persists in the Fallopian tubes of ovariectomized females treated with progesterone. The ability of the uterus to bring about complete capacitation is lost in the progesterone-dominated female. Sperm are completely capacitated in about 11 hr. in the oestrous uterus, but only partial capacitation occurred during this period in the uterus of the ovariectomized rabbit. As judged by the time of penetration of eggs placed into the uterus at different times after insemination, sperm in the ovariectomized uterus did not acquire the capacity to fertilize for approximately 20 hr. Injection of up to 1000 i.u. human chorionic gonadotrophin at the time of insemination did not result in significant depression of capacitation in the uterus. It is concluded that the rabbit oviduct has a greater innate potential for capacitation than the uterus, though this potential is increased in both sites by oestrogen stimulation. In contrast to its depressive effect on capacitation in the uterus, progesterone apparently does not significantly affect capacitation in the oviduct, even in the absence of the ovaries. This implies that modification of the endocrine status of the female rabbit would not produce a total contraceptive block of capacitation per se.