The metabolism of tritiated leucin has been studied in normal and Li-treated embryos of Xenopus laevis. The eggs were labelled by injecting tritiated leucin into the female toad during oogenesis. A decrease of the radioactive protein-bound leucin was observed during gastrulation. This decrease was more evident in ectoderm and mesoderm but less evident in entoderm (Table I, fig. 2). The loss of tritiated leucin from labelled proteins was greater in the control than in Li-treated embryos (Table I, figs. 1, 2).