The teratogenic activity of a stable analogue of thalidomide, 2( 2,6-dioxopiperiden-2-yl)-phthalimidine ( EMIz), has been compared in monkeys, rabbits, and rats. After oral administration of 25-250 mg/kg, EMIZ was teratogenic in monkeys and rabbits but only slightly so in rats. After iv injection of the compound dissolved in dimethyl sulfoxide and water, teratogenic effects were observed both in offspring of rabbits and rats receiving doses of 10 mg/kg. The malformations observed in monkeys and rabbits were identical with those observed after thalidomide treatment, but those in rats were not the typical thalidomide-type deformities.