The c-mos proto-oncogene exists as a maternal mRNA in mammalian oocytes, in that it has been shown to accumulate in mouse oocytes during the growth phase and to be present at high levels in fully grown oocytes. The function of c-mos during the subsequent development of the oocytes and embryos was examined by determining the fate of the oocyte c-mos mRNAs by in situ hybridization and Northern blot hybridization analysis. A substantial decrease in the levels of c-mos transcripts was observed in oocytes undergoing meiotic maturation. By the two-cell stage, levels of c-mos transcripts dropped to below the limits of detection using in situ hybridization. c-mos transcripts remained undectable through the blastocyst stage of embryogenesis. Analysis of meiotic maturation in vitro permitted finer temporal resolution of the initial drop in c-mos levels. Between approximately 7 and 17 h of culture, the amount of c-mos mRNA fell to 18-43% of the levels found in the fully grown oocyte. This interval corresponds to the progression of meiotic maturation from metaphase I to metaphase II. Our in vivo studies showed that ovulation per se is not the stimulus for the drop in c-mos transcript levels, since preovulatory metaphase II oocytes exhibited this decline to a degree comparable to that of ovulated metaphase II oocytes. The development specificity of c-mos transcript levels suggests a role of this putative serine kinase in the meiotic maturation of mammalian germ cells.