Cocaine abuse by pregnant women is increasingly recognized as causing serious health consequences for mother and newborn. To assess the placental transfer and fetal effects of cocaine, we studied its pharmacokinetics following intravenous administration to the pregnant ewe and fetus. Following bolus doses of 0.5-4.0 mg/kg to ewes, cocaine appeared within 30 s in fetal circulation, with peak concentrations occurring in 4-5 min. The disappearance of cocaine in the fetal plasma paralleled that in maternal plasma, suggesting that a rapid equilibrium of cocaine occurred between maternal and fetal compartments. The mean half-life of cocaine in the fetus across doses (4.4-5.0 min) was similar to that in the ewe (4.0-5.6 min). Plasma clearance of cocaine in the ewe did not appear to vary according to dose. The fetal exposure to cocaine, as indicated by the area under the fetal plasma concentration versus curve, was a linear function of maternal cocaine dose (r = 0.96, p less than 0.01). These results demonstrate rapid placental transfer of cocaine after maternal administration in an animal model and rapid metabolism by mother and fetus.