This document is intended to aid in the design and assessment of studies concerned with exploring the association between exposure to chemicals during pregnancy and defective development. This document includes a description of the use of laboratory data in defining the potential embryotoxic hazards of chemicals and methods of assessing the human risks associated with occupational or environmental exposure to such chemicals. Three major areas are covered including: prenatal toxic manifestations, postnatal manifestations, and short-term tests. This document primarily deals with chemical exposures during pregnancy and should be useful to all those concerned with the evaluation of chemical safety.