Changes in protein composition of the nuclear lamina were monitored during early development in Xenopus. Lamin LIII, the only lamin present in oocyte nuclei, serves as a lamin pool for the formation of pronuclei and early cleavage nuclei. It is present in embryos up to the tail bud stages. Lamins LI and LII, the lamins originally found in adult cell nuclei, appear at characteristic times in development. LI first appears at the midblastula transition (MBT), and LII at the gastrula. Tryptic peptide analysis revealed that all three lamin forms found in the embryo are identical with the adult lamins. De novo synthesis of LIII and LI, observed at MBT, is independent of transcription and must therefore be due to activation of maternal mRNAs. These results are discussed in relation to other nuclear changes occurring during early development.