Maternal RNA of mouse eggs and embryos was labeled by exposure of growing ovarian oocytes to ³H‐uridine in vivo 8 to 16 days before ovulation and fertilization. Labeled embryos from the 1‐cell stage to the blastocyst stage were collected, fixed, and autoradiographs of plastic sections prepared. The observed grain density was similar in the pronuclei and in the cytoplasm of 1‐cell embryos. Knowing the volumes of nucleus and cytoplasm, it was determined that 3% of the maternal RNA was found in the pronuclei. It is suggested that some of this nuclear RNA may be stable small nuclear RNAs (e.g. U1 RNA) retained from the germinal vesicle stage through meiotic maturation. During the 2‐cell stage and beyond, maternal RNA is degraded and labeled precursor is reincorporated into nuclear RNA, making it difficult to accurately quantitate the amount of nuclear maternal RNA. It is known that about one third of the total maternal RNA is lost between the 8‐cell and blastocyst stages. It was found that cytoplasmic grain densities in inner and outer cells of the morula and blastocyst were not significantly different. Thus, the loss of maternal RNA does not proceed more rapidly in the differentiating trophoblast than in the inner cell mass.