In this teaching laboratory, the students are directed in an exercise that involves designing and performing an experiment to determine estrogen's role in regulating delayed implantation (diapause) in female rats. To encourage active participation by the students, a discussion question is provided before the laboratory exercise in which each student is asked to search the literature and provide written answers to questions and to formulate an experiment to test the role of ovarian estrogen in inducing implantation in female rats. One week before the laboratory exercise, students discuss their answers to the questions with the instructor to develop an experiment to test their hypothesis that estrogen is involved with inducing implantation in the rat. A rat delayed implantation model was established that utilizes an estrogen receptor antagonist (ICI 182,780), which inhibits the action of ovarian estrogens. Groups of mated females are treated with either carrier (control) or ICI 182,780 (ICI) every other day, starting on "day" 2 postcoitus (pc) until "day" 8 pc. One-half of the females receiving ICI are injected with estradiol-17 on day 8 pc to induce implantation 4 days after the controls. If the ICI-treated females are not administered estradiol, embryo implantation occurs spontaneously ~4 days after the last ICI injection on "day" 8. This is a very simple protocol that is very effective and provides an excellent experiment for student discussion on hormone action and the use of agonists and antagonists.