Nitrous oxide and halothane, alone and in combination, have been repeatedly shown to cause fetal anomalies and increased fetal death rates in experimental animals. Epidemiologic studies dealing with pregnant operating room personnel who were chronically exposed to trace amounts of nitrous oxide and halothane have indicated an increased number of miscarriages amoung these women compared with women employed outside the operating room. Dental personnel are exposed to even higher concentrations of nitrous oxide in the dental operatory than those levels foun in the operating room. More numerous and more encompassing studies are required. Meanwhile, the dentist has a primary responsibility to give careful consideration to the use of inhalation anesthetics when planning or administering treatment to a pregnant patient, especially one in the first trimester. In addition, the profession must recognize the potential hazard to both pregnant dentists and pregnant dental assistants in this early stage of pregnancy. Of course, the dentist should always consider the developing fetus before using or prescribing any pharmacologic agents.