In our previous studies, it was found that activities of maternal peripheral lymphocytes and thymocytes were depressed during the implantation period in rats and rabbits. This study was therefore attempted to clarify further this immunosuppression locally by determining lymphocyte response in lymph nodes draining the uterus (DLN) and to elucidate the mechanism by which prostaglandin E (PGE) modulates immune response during the implantation process in rats. As compared with non-pregnant rats, the response of DLN lymphocytes to concanavalin A (Con A) was depressed during the implantation period in 100% of rats studied. The activity of DLN lymphocytes depressed on day 8 of pregnancy was, however, restored partially by the treatment of indomethacin (ID), indicating that prostaglandin (PG) might be one of factors responsible for immunomodulation during the process of implantation. DLN lymphocyte activity in non-pregnant rats was suppressed if PGE was pre-treated prior to Con A and this suppression was partially restored by the treatment of ID. Furthermore, DLN lymphocytes pre-treated with PGE produced PGE in vitro and this PGE production was blocked by the treatment of ID, suggesting that PGE induced PGE-producing cells. However, the pretreatment of estradiol, progesterone, and hCG at doses enough to suppress lymphocyte activity was ineffective in inducing PGE-producing cells. From these results, it is suggested that PGE induces PGE-producing suppressor cells, thereby increasing PGE concentration and PGE in turn depresses maternal local immune response as well as systemic immune response during the implantation period in rats.