The distribution of 2 4 dichlorophenoxyacetic acid 2 4 D was examined in maternal and fetal rabbits Pregnant New Zealand rabbits 28 30 d gestational age were anesthetized with ketamine xylazine and the femoral vein and artery were catheterized for compound 14 administration and sampling Dams received iv C 2 4 D 12 5 Ci kg with unlabeled sodium 2 4 D 1 10 or 40 mg kg in saline Blood and tissue were collected up to 2 h after dosing Fetal to maternal plasma AUC ratios were 0 09 0 07 and 0 16 after the 1 10 or 40 mg kg dose respectively Extraplasma AUCs were greatest in maternal kidney and uterus and lowest in maternal and fetal brain A greater than fourfold elevation in fetalAUC was found when the dose was increased from 10 to 40 mg kg suggesting satu ration of maternalplasma binding of 2 4 D Although the in vitro fetal brain tissue to incu bation media ratio was unity 1 03 0 1 mean SD fetal brain AUCs were 10 or less of the fetalplasma AUCs indicating the brain barrier system to 2 4 D is functioning in the late gestation fetal rabbit However its development may not be complete due to the higher brain tissue to plasma ratios in the fetus compared to the dam