The rate of transplacental passage of diazepam (DZ) has been studied in 33 cases of cephalic presentation where operative forceps delivery was indicated by intrauterine hypoxia or by prolonged second stage of labor. The drug (30 mg) was injected intravenously immediately before delivery either during uterine contractions (Group I) or in the relaxation period (Group II) according to a randomized protocol. As judged by the concentration in the newborn and the child/mother concentration ratio at 2 hr after delivery, and the concentration on the second day, the fetal exposure to the drug was probably less when the injection was timed to coincide with uterine contractions. In the group of patients given the drug in the relaxation period, the injection-delivery (I-D) interval was up to 305 sec and the 2-hr child/mother concentration ratio was close to unity in some cases. It therefore appears that the transplacental passage of DZ is rapid when the high initial concentrations in the maternal circulation coincide with favorable conditions for transfer in the relaxation period. Although sleep was induced by the injection of DZ in all of the mothers, the amounts of drug transferred during the short I-D intervals in the present study did not exert delterious effects on the newborn infants.