Coitus is a unique physiological event which may result in internal fertilization, whereby active allogeneic spermatozoa are presented to the female host which can invade her tissues. Sterile coitus on the other hand, stimulates the uterine cervix in the rat and induces a physiological state of pseudopregnancy which interrupts the normal pattern of the oestrous cycle and establishes a persistent dioestrous state. This state is hormonally controlled and in its early stages is not dissimilar from that of true pregnancy. Pregnancy hormones are known to influence the immunological status of the female. Thymic observations during early pseudopregnancy showed interstrain differences in weight, cell content and thymocyte proliferation. Pooled data indicated increased cell density on days 3, 4 and 5 of pseudopregnancy associated with a peak of mitotic activity on day 3 in both strains. The timing of this proliferative activity seems to reflect the needs of the immune system in its putative response to mating and implantation.