By use of Positron Emission Tomography (PET), dynamic studies of the metabolism within the feto-maternal unit can be performed using various tracers. Many compounds like amino acids, carbohydrates, fatty acids and drugs can be performed using various tracers. Many compounds like amino acids, carbohydrates, fatty acids and drugs can be labelled with 11C and used as tracers. 11C-labelled L- or D-methionine was injected intravenously into pregnant Rhesus monkeys. The distribution of the radioactivity in maternal muscles, aorta, placenta and the liver of the fetus was quantitatively estimated as a function of time. Simultaneously blood, urine and amniotic fluid samples were analyzed for 11C-activity. The distribution of 11C between the high and the low molecular fraction of plasma (MW greater than 5000) was studied after gel filtration. Both when 11C-L- and D-methionine were given, the radioactivity rapidly crossed the placenta and was accumulated in the fetal liver. In the 11C-L-methionine experiments, about 70 per cent of the radioactivity in plasma was found in the high molecular fraction one hour after injection. A greater part of 11C-D-compared to 11C-L-activity was excreted in the urine.