An active immunological response by the female to mating and pregnancy is important, if not essential, to successful reproduction. Alloantigens which have been demonstrated on spermatozoa include histocompatibility antigens. In the enigmatic success of the fetal allograft, the conceptus survives owing to failure of the mother either to initiate or complete the process of rejection. In this study, AO(RT1u/AgB2) female rats were caged with male rats of the same strain. Observations were obtained from the first 5 days of their postcoital period, identifying lymphoid tissue changes which are due to the presence of the ejaculate and early products of conception in the female reproductive tract. A significant increase in intrathymic proliferation by day 3 is thought to be a hormonal effect induced by the cervical stimulation at mating. The postcoital exodus of lymphocytes from the thymus is possibly in response to coitus and induced by the seminal plasma to safeguard the implanting conceptus.