Epilepsy is a chronic nervous disorder caused by abnormal electrical signals in the brain. Epileptic seizures can be minimized with the use of anticonvulsant drugs. Use of anticonvulsants by a pregnant woman poses the risk of damage to an unborn baby. One of the most common anticonvulsants is diphenyihydantoin (DPH).To study the effects on the fetus of preimplantation exposure to anticonvulsants, DPH was injected into A/J and NSA strains of mice during the first five days of pregnancy. Fetuses were examined externally on Day 18 of development and later dissected for internal examination. The DPH mice of both strains showed no internal or external anomalies other than reduced size. Other effects found were the inability to establish pregnancy, increases in resorptions, and smaller litter size.