Pregnancy was timed in cats following induced ovulation. Methotrexate, (0.5 mg/kg), aminopterin, (0.1 mg/kg), and acetylsalicylic acid, (25 or 50 mg/kg) were administered orally in gelatin capsules in single daily doses on different days of gestation, methotrexate (MTX) on days 11‒14, 14‒17, or 17‒20, aminopterin on day 12, 14, or 16, and acetylsalicylic acid (ASA) on days 10‒15 or 15‒20. Maternal toxicity was produced only by MTX. MTX given on days 11‒14 and 14‒17 produced high frequencies of malformations including umbilical hernia. Aminopterin caused no conclusive teratogenic response. An overall increased frequency of anomalies occurred after 50 mg/kg ASA but no single anomaly predominated.