We have studied the accumulation and localization of U1 RNA during mouse embryo development by in situ hybridization with a U1 RNA probe and immunofluorescence microscopy using a mouse monoclonal antibody to U1 snRNP. There is a substantial amount of U1 RNA present in the oocyte that is present in both the germinal vesicle and the cytoplasm although the concentration is higher in the nuclear compartment. Following the germinal vesicle breakdown that accompanies ovulation and meiotic maturation, the U1 RNA is uniformly distributed throughout the unfertilized oocyte. In the fertilized egg, the silver grain density from in situ hybridization is higher over pronuclei and this enrichment is maintained at the two-cell and later stages. Similar results were obtained for the distribution of the U1 snRNP as assayed by immunofluorescence microscopy: U1 RNA is predominantly localized in all nuclei except polar body nuclei. The U1 RNA in the oocyte and two-cell embryo is predominantly (greater than 85%) U1a RNA. By the eight-cell stage there is a two to three-fold increase in the amount of total U1 RNA and the proportion of U1b RNA has increased to about 40%. The amount of U1 RNA continues to increase through the blastocyst stage and the proportion of the U1b RNA increases to 60%.