The placental permeability and transfer of lead nitrate alone and lead chelated with ethylenediaminetetraacetic acid (PbEDTA), nitrilotriacetic acid (PbNTA), iminodiacetic acid (PbIDA), and penicillamine (PbPEN) were determined. 210Pb(NO3)2 was infused into pregnant rats on day 18 of gestation at a rate of 0.5 mg/kg/min alone and with an equimolar amount of chelating agent. The results showed that the placental transfer of lead is increased with the infusion of PbNTA, PbPEN, and PbEDTA despite the fact that maternal whole blood and plasma lead concentrations were about one-third that with lead alone. Little or no differences in maternal blood lead content or fetal lead content were observed with the PbIDA complex. The administration of a single iv dose of lead alone (50 mg/kg) or with equimolar amounts of chelating agents resulted in approximately a 400% increase in fetal lead content with PbPEN and PbNTA and a 50% increase with PbEDTA by 4 hr after administration as compared to lead alone. During this time PbNTA, PbPEN, and PbEDTA had been mostly eliminated by the maternal animal so that no further increase in placental transfer occurred. The net result of increased permeability and maternal elimination was a fetal lead content similar to lead alone with PbPEN and PbNTA and a greatly reduced fetal lead content with PbEDTA at 24 and 48 hr after administration. It is concluded that the membrane permeability and the placental transfer of lead can be increased with lead-chelate complexes. Under the conditions of our experiments this factor is balanced by more rapid maternal elimination and faster termination of the fetal exposure to lead when present in its chelated form.