High serum gamma-glutamyl transpeptidase activity was found in cord blood of newborn boys. This activity decreased to lower values on the 4th and 7th days. In newborns treated for 3 days following the birth with a combination of phenobarbital and nicethamide an increase of gamma-glutamyl transpeptidase activity occurred from the 4th to the 7th days. The 7th day levels were significantly higher when compared with the controls. Simultaneous determination of urinary glucaric acid excretion confirmed the induction of hepatic microsomal enzymes in glucuronic acid pathway. This could also be demonstrated by a pronounced decrease of serum bilirubin levels in groups receiving the enzyme inducers whether phenobarbital was administered intramuscularly or orally as sodium salt solution.