The transport and localization of mRNAs within Xenopus oocytes contributes to an asymmetry that determines axis formation in the embryo. Several mRNAs become localized along the animal-vegetal axis. The mitochondrial cloud, a specialized cytoplasmic compartment which contributes to the formation of germ plasm, and the endoplasmic reticulum play important roles in sorting and localizing these RNAs. As in other species and cell types, the localization of RNAs in Xenopus oocytes is directed by signals in the 3' untranslated regions of the transcripts. Cytoplasmic proteins that interact with these signals are being identified, as are the links between RNA localization, intracellular organelles and early embryonic development.Copyright 1997 Academic Press Limited.