Although the importance of methyl metabolism in fetal development is well recognized, there is limited information on the dynamics of methionine flow through maternal and fetal tissues and on how this is related to circulating total homocysteine concentrations. Rates of homocysteine remethylation in maternal and fetal tissues on days 11, 19, and 21 of gestation were measured in pregnant rats fed diets with limiting or surplus amounts of folic acid and choline at two levels of methionine and then infused with L-[1-C-13, H-2(3)-methyl] methionine. The rate of homocysteine remethylation was highest in maternal liver and declined as gestation progressed. Diets deficient in folic acid and choline reduced the production of methionine from homocysteine in maternal liver only in the animals fed a methionine-limited diet. Throughout gestation, the pancreas exported homocysteine for methylation within other tissues. Little or no methionine cycle activity was detected in the placenta at days 19 and 21 of gestation, but, during this period, fetal tissues, especially the liver, synthesized methionine from homocysteine. Greater enrichment of homocysteine in maternal plasma than placenta, even in animals fed the most-deficient diets, shows that the placenta did not contribute homocysteine to maternal plasma. Methionine synthesis from homocysteine in fetal tissues was maintained or increased when the dams were fed folate- and choline-deficient methionine-restricted diets. This study shows that methyl-deficient diets decrease the remethylation of homocysteine within maternal tissues but that these rates are protected to some extent within fetal tissues.