The mechanisms of the fetal transmission of vitamin A were investigated by injecting 125I-retinol-binding protein (RBP), 131I-prealbumin and 3H-retinol-RBP in 9 pregnant rhesus monkeys. Samples of blood, amniotic fluid, and when needed fetal liver and kidney were collected after 2.5–48 h. 125I-RBP and 131I-prealbumin were detected in fetal plasma and amniotic fluid already in the first samplings. The specific radioactivity of fetal RBP increased during the first 25 h but never exceeded 13% of the maternal value. This is presumably due to concurrent synthesis of RBP (and prealbumin) by the fetus. The maximum specific activity of 125I-RBP in amniotic fluid was 70% of the maternal value indicating that the protein was predominantly derived from the mother. 3H-retinol was more readily transmitted to fetal plasma than RBP. A significant portion of the 3H activity was found in the lipoproteins suggesting that some retinol enters the fetal circulation by other routes than those involving transmission of RBP. The high specific activity of retinol in the fetal kidney implicates its direct involvement in the turnover of transmitted vitamin A. Quantitatively, however, the accumulation of vitamin A activity was largest in the fetal live