Implantation in mammals requires the successful completion of a series of integrated phenomena, including uterine preparation, synchronized embryo transport, embryonic attachment, uterine transformation, placental development, and the requisite hormonal milieu to support each step. Potential for toxic interference with early pregnancy exists at several points in the course of events via a variety of anatomical and physiological sites. An improved understanding of the mechanisms of implantation failure due to toxic insult is necessary in order to assess risks of reproductive toxicants to the human female population. As an approach to providing such information, a panel of tests has been assembled and developed to probe the mechanisms by which chemicals affect fertility in rodents. These assessments are performed only if adverse effects on litter size or pregnancy are evident from previous reproductive studies. The evaluation of methoxychlor, a weakly estrogenic pesticide, has served to partially validate the panel. The early pregnancy protocol provides does-response information on the effects of short-term exposure of animals to compounds during early pregnancy. (Copyright (c) 1990 by the Society of Toxicology.)