AbstractThe teratogen 2‐amino‐l,3,4‐thiadiazole (ATDA) is an antagonist of nicotinamide, and may act by interfering with the synthesis or utilization of the coenzyme nicotinamide adenine dinucleotide (NAD). Several compounds that can be converted to NAD were tested as antiteratogens against ATDA. At day 11 of gestation (sperm day = day 0) pregnant Wistar‐derived rats were given single ip injections of ATDA (100 mg/kg), or ATDA immediately followed by the suspected antiteratogen ip or by gavage, or the antiteratogen alone. Compounds tested were NAD, nicotinamide, nicotinic acid, quinolinic acid, kynurenine sulfate, and L‐tryptophan, in doses of 10–200 mg per animal. At autopsy (day 20) fetuses were recovered and examined. It was found that each antiteratogen significantly reduced the frequency of ATDA‐induced resorptions and malformations. At certain doses each antiteratogen gave complete protection against ATDA‐induced malformations in some, but not all, litters. These results support the hypothesis that ADTA interferes with the synthesis or utilization of NAD and suggest that substances converted to NAD act as antiteratoge