THE CHICK EMBRYO is used widely in studying drugs for teratogenic activity. Chemicals, which have been injected at the same period of incubation of the chick embryo, may produce separate and characteristic patterns of developmental abnormalities. The effects of many drugs, such as insulin, azaserine, sulfanilamide, 4-aminopteroyl glutamic acid, 8-azaguanine, physostigmine, thallium, lead, boric acid and cortisone, have been reviewed. The chick embryo in the egg is an isolated and independent system, whereas mammalian embryos are usually intimately involved with the maternal host which may detoxify, excrete or otherwise protect the fetus against noxious chemicals. It was, therefore, of interest to determine the effects of drugs, teratogenic in the chick embryo, on the mammalian fetus. The rat was selected because of its availability and because there is considerable information on rat embryology and teratology. Because some drugs, apparently inactive in the chick embryo, have produced consistent developmental abnormalities in the rat fetus, the pregnant rat also has been used for the initial study of selected compounds. The objectives of these studies were: 1) to determine and compare the teratogenic action of drugs on the chick and rat embryos; 2) to determine the consistency and specificity of action of each drug; 3) to determine the relation between the time during gestation when a drug is introduced and the occurrence of specific abnormalities; 4) to determine the relationship of the dose of a drug toxic to the mother, the dose toxic to the embryo, and the dose producing consistent developmental defects, and 5) to detect compounds which protect the embryo from the teratogenic action of a chemical.