Two cleavage stage embryos, two blastocyts surrounded by zona pellucida, and two azonal ('hatched') blastocysts were recovered from uteri of rhesus monkeys and examined with the electron microscope. Cleavage-stage embryos showed no striking differences between inside and outside blastomeres, all of them displaying primitive junctional complexes, heteromorphic mitochondria, large ovoid nuclei and a few polyribosomes. The zonal blastocysts had distinctive trophectoderm and inner cell mass areas in which tight junctional complexes and occasional desmosomes were observed at cell membrane junctions. Such embryos also contained numerous polyribosome complexes and cisternae of rough endoplasmic reticulum, while some inner cell mass cells displayed large autophagic vacuoles. Hatched blastocysts showed an increased number and length of microvilli on the free surface of the embryo, and the trophectoderm cells contained microfibrillar bundles not seen in equivalent regions of zonal blastocysts. Most of the listed ultrastructural features of the rhesus preimplantation embryo are also present in similar embryos of other mammalian species.