In this paper, data are presented on the characterization of uterus draining lymph nodes (UDLN) B and T lymphocytes of virgin rats at estrus (E) and diestrus (D). We established that the T/B lymphocyte relationship in the UDLN is less than one at estrus and more than one at diestrus. This is due to a decrease in the percentage and in the total number of mature T cell population in association with a decrease in the percentage of the OX8 subset in the UDLN at estrus. This situation may be related to an increase in estrogens known to produce thymic involution. These changes were not observed when we studied peripheral (popliteal) lymph nodes. The changes observed in the UDLN T cell population at estrus could be under hormonal control and we think that this condition may be important to prepare the immune system for an eventual pregnancy.