In the present study, hormonal changes in uterine tissue and circulation were evaluated during the implantation period in rats in order to understand the mechanism by which implantation takes place. The results obtained were as follows. 1. Concentrations of serum estradiol and progesterone were significantly increased on days 4 and 5. 2. Concentration of estrogen receptor reached maximum on day 5 when implantation normally occurred in rats. On the other hand, progesterone receptor was gradually decreased, reaching the lowest on day 5. 3. Uterine PGs and cAMP concentrations were significantly increased on day 5. 4. Uterine PGs and cAMP concentrations in implant sites were significantly greater than those in non-implant sites. It is, therefore, concluded that prostaglandins and cAMP in uterine tissue as well as circulating ovarian steroid hormones were increased during the period of implantation, suggesting that these hormones might be actively involved in the process of implantation in rats.