P lacental transport of vitamin B₁₂ was studied in the pregnant rat in two series of experiments. In the first series animals were given cyanocobalamin-⁵⁷Co intravenously at various stages of gestation. High specific activity tracer was used and doses of B₁₂ were 1-2 ng per animal. The rats were killed from 15 min to 24 hr after injection and the fetuses, placentas, and serum were assayed for radioactivity. In the second series using uninjected animals, absolute amounts of vitamin B₁₂ in fetuses and placentas were measured at stages of gestation from day 12 through day 20.There was a progressive increase in B₁₂ transferred to the fetus during gestation. Although the quantity of vitamin B₁₂ transported per 24 hr was proportional to fetal weight, the amount transported per gram of placenta increased tenfold from day 10 through day 19. Uptake of tracer B₁₂ by placenta was initially rapid; however, no radioactivity appeared in the fetus until 2 hr after injection. The actual amount of B₁₂ in placenta increased throughout gestation, and the placental concentration of B₁₂ was greater than maternal plasma and fetal tissue concentrations at all times measured.These data suggest that the ability of placenta to transport B₁₂ increased throughout gestation, and that the rate-limiting step in the transport process was either the passage of B₁₂ from the maternal to the fetal side of placenta or the transfer from placenta into fetal plasma.