SUMMARY The capacity of rabbit spermatozoa to penetrate the zona pellucida of the ovum has been reported to depend upon pre-conditioning within the reproductive tract of the oestrous female. The present experiments show that spermatozoa can be capacitated in the uteri and uterine tubes of rabbits ovariectomized 1 month previously, in the isolated colon or bladder of male and female host rabbits, and in the anterior chamber of the eye and glandula vesicularis of male hosts. Spermatozoa were partially capacitated when they were confined within the excised uterine horns of a rabbit, and when they were suspended with strips of endometrium in saline solution at room temperature. Capacitation was not obtained when spermatozoa were confined in a cellophane dialysis bag within the intact uterus of an oestrous host, in the excised uterus of a sheep, or when they were suspended in saline with red blood cells or with dead spermatozoa.