To investigate whether fetal acidosis increases the placental transfer of lidocaine resulting in higher fetal blood levels of the drug, lidocaine was infused i.v. into 10 pregnant ewes to maintain plasma levels of 2-4 .mu.g/ml. After maternal-fetal equilibrium was reached, the fetus was made acidotic by infusing lactic acid i.v. Fetal blood pH decreased from 7.35 to 7.10. With fetal acidemia, fetal blood lidocaine levels increased significantly from 1.60 .+-. 0.11 .mu.g/ml to 2.72 .+-. 0.26 .mu.g/ml. The fetal-maternal lidocaine ratio increased from 0.76 to 1.21. Correction of the acidosis by H-CO infusion returned the fetal-maternal ratios to control values. Acidosis in the fetus might result in trapping of ionized lidocaine in the fetal circulation and increase the transfer of lidocaine across the placenta.