Fertilizations by diploid rabbit spermatozoa cannot be precluded on grounds of either reduced viability or abnormal functional anatomy of head structures. However, the cervix has been shown to constitute a major barrier to the progression of diploid spermatozoa to the site of fertilization. The almost total absence of double-tailed spermatozoa from the cervix indicates that the selectivity of sperm transport is most probably based on differential sperm motility. This finding is supported by the failure of detergent-killed spermatozoa to penetrate the cervix, and also by in vitro experiments using systems in which the distribution of spermatozoa is affected by their swimming abilities. There is no evidence for any active selection of spermatozoa by the female tract. Therefore those diploid spermatozoa whose only apparent fault is their larger size would seem to be potentially capable of fertilizing. So although the actual involvement of diploid spermatozoa in the production of triploid zygotes in the rabbit is still unknown, the possible incidence of such diandric triploids will be much less than the simple incidence of diploid spermatozoa in the semen. Studies of sperm transport in the human female tract have shown that thus apparent selection for spermatozoa of normal morphology can be attributed to the general principle that those spermatozoa with abnormalities which impair their motility are excluded from the cervix, whilst those with abnormalities which do not affect motility will have the same chance of reaching the site of fertilization as 'normal' spermatozoa. Again the spermatozoa select themselves, with the female tract playing an essentially passive role. Ultrastructural studies on rabbit and human spermatozoa have emphasized the stability and complexity of fine structure of the postacrosomal sheath. These findings are considered in relation to a possible role for this region in the maintenance of the structural integrity of the sperm head.