Abstract: It was recently reported that managanese can inhibit spontaneous in vitro nuclear maturation in bovine cumulus-enclosed oocytes (CEO). The goals of the present study were to: (1) determine if the effect of manganese on nuclear maturation is dependant upon protein kinase A (PKA) activity; (2) compare the effecs of manganese and activ ation of adenylate cyclase with forskolin (FSK) on nuclear maturation in denuded oocytes (DO) and CEO and cAMP level sin DO, cumulus-oocyte complexes (COC) and oocytes derived from COC; and (3) determine if protein kinase C (PKC) activation can reverse manganese-maintained meiotic arrest in bovine CEO. The PKA inhibitor H-89 significantly decreased the percentage of CEO maintained at the germinal vesicle (GV) stage by manganese during a 9-h culture period (1, 82, 14 and 37% GV for control, 0.05 mM MnCl2, 125 ?M H-89 and mnCl2 + H-89 respectively). The adenylate cyclase activator FSK and MnCl2 had the same ihbibitory effect on nuclear maturation in CoC; MnCl2 significantly increase cAMP levels in COC but to a lesser extent than FSK. In Do, MnCl2 significantly decreased the percentage of oocytes maintained at the GV stage by FSK after 7 h of culture; however, neither agent had a significant effect on cAMP levels compared to control oocytes. The PKC activator PDD B reduced the percentage of CEO maintained in meiotic arrest by MnCl2. In conclusion, the likely mechanism of action of MnCl2 to maintain meiotic arrest in CEO is by activation of the adenylate cyclase enzyme in cumulus cells to generate cAMP which would activate PKA.