Methotrexate (MTX), administered as a single 20 mg/kg intraperitoneal dose to C57Bl/6 J mice on their 9th day of pregnancy results in high incidences of median facial clefts in the surviving gestational-day-18 fetuses. We have shown the presence of dilated and congested blood vessels in the frontonasal prominences (FNP) of embryos from treated mothers as early as 3 hours following drug administration. Within 24 hours, large vascular blebs are located in the FNP and the neural tubes appear somewhat distended. By 32 hours after treatment, distention of the neural tube is marked while blebs have become less evident. Subsequent to these changes, FNP mesenchymal deficiency as well as neural tube distention lead to the formation of median facial clefts. It is hypothesized that, as with a number of other teratogenic agents (especially hypoxia), initial fluid imbalance is the primary teratogenic insult.