BMP-7 is an important mediator of metanephric mesenchyme differentiation during kidney development. Gene knockout studies have shown that BMP-7 null mutation mice die shortly after birth due to renal failure, although the induction of metanephric structures has initially occurred (E11–E13). Iodinated BMP-7 was injected into the tail vein of pregnant mice and its availability to tissues and fetuses was further analyzed by tissue uptake, LM autoradiography and SDS-PAGE electrophoresis. Studies on the distribution of 125 I-BMP-7 injected through the tail vein of pregnant mice indicated that 125 I-BMP-7 passed across the placenta and localized in developing fetal organs, in particular kidneys, up to day 14 of gestation. At later stages of pregnancy 125 I-BMP-7 did not pass the trophoblasts of the placental barrier and did not enter the fetal blood vessels. The analysis of the distribution of 125 I-BMP-7 from pregnant mice to fetal organs, in particular the kidney, suggests a cross-over of maternal circulating BMP-7 to the fetus through the placental barrier.