Four pregnant Rhesus monkeys were given a 1 week protein restricted diet with the objective to monitor possible effects on transplacental transfer of [11CH3]-L-methionine with positron emission tomography (PET). The weight was reduced in all monkeys, two of which later had an intrauterine fetal death. The mean venous serum concentrations of free methionine in the mothers were not significantly changed during the study period, although a tendency towards lower values during protein restriction was noted like in the majority of the amino acids. The uptake of trace amount of [11CH3]-L-methionine in the fetus was studied with PET before the reduced diet, after 1 week on the reduced diet and after the return to a habitual diet. Our results indicate an increase in the uptake of radioactivity in the amniotic cavity after protein restriction. The PET-technique in its present form seems to be a useful method in the investigation of transplacental transfer of several natural amino acids as well as of other compounds labelled with 11C.