Compounds differing from thalidomide in either the phthalimide or the 2,6-dioxopiperidine moiety of the molecule were synthetized and tested for teratogenic potency in White New Zealand rabbits. Both the 2,6-dioxopiperidine and 2-oxopiperidine derivatives of phthalimide and phthalimidine were found to be highly teratogenic. A somewhat higher teratogenic potential appeared to be associated with the 2,6-dioxopiperidine derivatives. The most potent teratogen investigated was clearly 3-(1,3-dihydro-1-oxo2H-isoindol-2-yl)-2,6-dioxopiperidine (EM12). Compounds in which the phthalimide ring was replaced by 2,3-dihydro-1,1-dioxido-3-oxo-1,2-benzisothiazol, did not induce any embryopathic effect differing from control data. No consistent correlation between teratogenic activity and sedative properties of the compounds was detected. The results are discussed in respect to current views of the molecular mechanism leading to thalidomide embryopathy.