The total RNA content of mouse oocytes, as measured by ethidium bromide fluorescence, was found to decrease by 19% during meiotic maturation (ovulated eggs contain 19% less RNA than full-grown oocytes). Consistent with these results, prelabeled stable RNA of full-grown oocytes decreased by about 20% during in vitro maturation. Polyadenylated RNA represented 19% of total prelabeled RNA in full-grown oocytes and 10% in oocytes matured in vitro, confirming previous results on in vivo prepared material. To distinguish between deadenylation and degradation for one mRNA, the amount and state of adenylation of actin mRNA was examined using Northern blots of oocyte RNA probed with a nick-translated β-actin cloned chicken cDNA. The results showed that the amount of actin mRNA remained similar during maturation, but its molecular weight decreased slightly. Experiments in which RNA was treated with oligo (dT) and RNase H demonstrated that the actin mRNA was deadenylated during maturation, when actin synthesis is known to decline. These results indicate that the previously defined loss of bulk RNA and changes in the state of adenylation of mRNA during the first 1 1 2 days of embryogenesis actually begin during the 12 hr of meiotic maturation preceding fertilization.