Old paths retrodden.
Much new information is present about the origins of developmental disorders resulting from chemical exposure. However, there is a danger of losing sight of the precepts and principles established in the light of existing knowledge. Our ability to identify risks to normal development and reproduction would be much improved if a number of straightforward rules were always followed. This article proposes 10 precepts: do not neglect resorptions; use appropriate terminology; study pathogenetic mechanisms; strive to develop new models of evaluating data; slavish adherence to guidelines blocks informed ratiocination; do not forget the yolk sac; remember the adaptability of pregnant women; respect the environment of the unborn; avoid causing unnecessary alarm and do not confuse association with causation; and finally, always remember the human purpose for which tests on laboratory animals are performed. These tenets are emphasised in order that in pursuing new approaches and integrating new information we do not lose sight of existing knowledge.