In thirteen newborn infants of mothers affected with "petit mal" and treated with ethosuximide, submitted to artificial feeding with powdered milk, it was found that transplacental elimination of the drug occurred. Authors recorded daily serum concentration of ethosuximide and found that average life before it was totally eliminated was 80.62 +/- 7.24 hours. At the moment of birth serum concentration of mothers, and their newborn infants are similar due to transplacental by-pass. Through a lineal regression coefficient they find a relation between serum concentration in newborn infants and time passed from the moment of birth. This coefficient was negative and thus signified facility of the newborn to eliminate the drug. This result allows us to treat the mother adequately during anti-natal period with adequate dose to avoid subsequent epileptic attacks without producing toxicity in the newborn child.