Radioactive methylmercury given to rhesus monkeys in late pregnancy crossed the placenta slowly from mother to fetus. The maternal-fetal ratio in both plasma and erythrocytes was as low as 10:1 serveral hours after maternal administration. Transfer from fetus to mother was even slower with the concentration gradient remaining above 25:1. Organic mercury was generally distributed throughout fetal tissues in a similar manner regardless of the routine of administration. The placenta and blood-brain barrier each appear to represent partial impediments to acute transfer of organic mercury, probably because of extensive erythrocyte binding. While the hemochorial placenta, by virtue of its limited permeability to organic mercury, appears to afford the fetus some degree of protection of acute mercury poisoning the fact that mercury moves out of the fetel circulation even more slowly than into it probably accounts for the fetal hazard with chronic exposure.