Administration of estradiol-17β to male Xenopus laevis resulted in an elevation of 3H-leucine incorporation into soluble proteins of the liver within 12 hr although the specific activity of the total serum proteins of these animals remained unchanged during the 36 hr in which this study was conducted. However, isotopic immunochemical analyses of the labeled serum proteins revealed that lipovitellin, a yolk protein synthesized in the liver, is released into the serum within 12 hr after administration of estradiol-17β. These data along with results already published suggest that secretion of lipovitellin occurred within 3 hr following induction of protein synthesis by estradiol-17β. (Endocrinology93: 256, 1973)