Iliac and popliteal lymph nodes were removed from Sprague-Dawley female rats killed at 12-13 weeks of age in groups of 10 animals: one group of virgin controls and 10 groups at intervals of two days until the tenth day after either 'inbred' or outbred mating. The iliac lymph nodes on the second post-coital day after outbred mating were significantly heavier than those of virgin animals and on the fourth postcoital day were significantly heavier after outbred mating than 'inbred' mating. The total lymphocyte counts in the iliac and popliteal lymph nodes were significantly less in the virgin animals than those made on ;the second post-coital day after both 'inbred' and outbred mating. These increased counts were sustained until the tenth post-coital day and were associated with significantly increased lymphocyte cell densities in the nodes. Such lymphocyte accumulation probably results from an endogenous or exogenous hormonal stimulus. The proportions of proliferating cells in the lymphocyte population of the iliac and popliteal lymph nodes from the fourth until the tenth post-coital day were significantly greater after outbred mating than after 'inbred' mating. These observations indicate that after outbred mating the female animal mounts an immune response to the allogeneic spermatozoa and semiallogeneic products of conception within her reproductive tract.