The detection of the teratogenic effect of drugs remains a challenge. The sudden appearance of the problem has baffled pharmacologists. The testing of the teratogenic action of a new drttg requires a systematic and complete investigation, without bias, based on the chemical s1rttctttre or pharmacologic action. The absence of consistent sensitivity in animal species and of a valid reference standard has slowed up progress in solving the problem. We hope, however, that with research, the exchange of pharmacologic information, and the continuation of cooperation between pharmacologists, biochemists, geneticists, and clinicians, the teratogenic effect of a new drug will eventually become a calculable risk.