Estradiol replacement to ovariectomized female rats causes dramatic changes in hippocampal structure and function as well as in performance on hippocampally dependent tasks. Using a delayed matching-to-place version of the water maze, the present study examines the time course of estradiol-induced enhancements in memory retention as well as the effectiveness of acute and continuous patterns of replacement. One 10-microg injection of estradiol administered on each of two successive days resulted in significant improvements in memory retention that persisted for approximately 4 days following the second injection. When estradiol administration continued for 10 consecutive days, these improvements in memory retention persisted. These findings indicate that estradiol replacement can improve memory retention and that these improvements can be maintained by continuous replacement for at least 10 days.