Rates of protein synthesis were measured in whole fetuses and maternal tissues at 17 and 20 days of gestation in postabsorptive rats using continuous infusion of L-[1-14C]leucine. Fetal protein degradation rates were derived from the fractional rates of synthesis and growth. Whole-body (plasma) leucine kinetics in the mother showed a significant reduction of the fraction of plasma leucine oxidized in the mothers bearing older fetuses, a slight increase in the plasma flux, with total leucine oxidation and incorporation into protein remaining similar at the two gestational ages. Estimates of fractional protein synthesis in maternal tissues revealed an increase in placental and hepatic rates at 20 days of gestation, whereas the fractional synthetic rate in muscle remained unchanged. A model for estimation of the redistribution of leucine between plasma and tissues is described in detail. This model revealed a more efficient utilization of leucine in fetal protein synthesis in comparison with other maternal tissues, a greater dependency of the fetus on plasma supply of leucine, and a significant increase (2-fold) in the release of leucine from maternal muscle as the fetal requirements increased proportionately with its size. The latter conclusion, supported by nitrogen analysis and the ratio of bound-to-free leucine in maternal tissues, confirms the importance of maternal stores in maintaining the homeostasis of essential amino acids during late pregnancy.