Activation of members of the MAPK family, Erk 1 and 2, in oocytes resuming meiosis is regulated by Mos. The cAMP-dependent PKA-mediated cAMP action that inhibits the resumption of meiosis also prevents MAPK activation. We hypothesized that PKA interferes with the MAPK signaling pathways at the level of Mos. We also assumed that this regulatory cascade may involve p34cdc2. To test our hypothesis we explored the role of PKA and p34cdc2 in regulating Mos expression. Rat oocytes that resume meiosis spontaneously served as our experimental model. We found that meiotically arrested rat oocytes express the c-mos mRNA with no detectable Mos protein. The presence of Mos was initially demonstrated at 6 h after meiosis reinitiation and was associated with its mRNA polyadenylation. (Bu)₂cAMP inhibited Mos expression as well as c-mos mRNA polyadenylation. Both these cAMP actions were reversed by the highly selective inhibitor of the catalytic subunit of PKA, 4-cyano-3-methylisoquinoline. Polyadenylation of c-mos mRNA was also prevented by roscovitine, which is a potent inhibitor of p34cdc2. Ablation of MAPK activity by two specific MAPK signaling pathway inhibitors, either PD 98059 or U0126, did not interfere with Mos accumulation. Our results suggest that translation of Mos in rat oocytes is negatively regulated by a PKA-mediated cAMP action that inhibits c-mos mRNA polyadenylation and involves suppressed activity of p34cdc2. We also demonstrate that stimulation of Mos synthesis in the rat does not require an active MAPK.