AbstractThe quantitative changes in the mRNAs for ribosomal proteins L7a, L18a, and S15 were assayed in slot hybridization experiments using labeled cRNA probes with total RNA from late growth‐phase oocytes, ovulated eggs, and early embryos through the blastocyst stage. All three mRNAs showed a similar developmental pattern of prevalence, but their copy numbers per oocyte or embryo fluctuated according to developmental stage. There are on an average about 17,000 copies of each mRNA in the late growth‐phase oocyte; this number drops to one‐fifth to one‐tenth in the ovulated egg and two‐cell embryo but increases rapidly during cleavage to about 25,000 in the eight‐cell embryo and about 42,000 in the blastocyst. A comparison of the levels of these mRNAs with the reported rates of ribosomal protein synthesis (LaMarca and Wassarman, 1979) suggests that, in late growth‐phase oocytes, ribosomal protein synthesis is regulated primarily at the translational level and is kept low by some factor limiting mRNA utilization. On the other hand, the high rate of ribosome biosynthesis during early embryogenesis from the two‐cell stage onward appears to involve the coordinate activation and transcription of ribosomal RNA and ribosomal protein genes coupled with the immediate translational utilization of ribosomal protein mRNAs.