The effects of drugs administered to pregnant women on bilirubin concentrations in 1,107 consecutively born infants are presented. Administration of narcotic agents, barbiturates, aspirin, chloral hydrate, reserpine, and phenytoin sodium all resulted in lowering of infant serum bilirubin concentrations. Diazepam and, to a lesser extent, oxytocin caused an elevation of infant serum bilirubin concentrations. Although many drugs were shown to alter serum bilirubin levels significantly, the clinical importance of such alterations was not dramatic except possibly in special circumstances. The phenothiazine derivatives, general or local anesthesia, sulfadimidine, ampicillin, and penicillin had no such effect on the newborn infant when given to the mother before delivery.