Rat oocytes resume meiosis spontaneously in vitro within 3 h after their isolation from the ovarian follicles. We report here that the spontaneous maturation of isolated rat oocytes is preceded by a drop in intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP). Further experiments were carried out to examine the possible correlation between the meiotic status and cAMP levels within the oocyte. To challenge rat cumulus-free oocytes to generate cAMP, bypassing their own adenylate cyclase, a preparation of an invasive adenylate cyclase from Bordetella pertussis was used. We found a dose-dependent elevation of cAMP levels within these oocytes that corresponded to inhibition of their spontaneous maturation. Persistent inhibition of meiosis was obtained with the continuous presence of the enzymatic preparation, whereas its removal resulted in a transient inhibition associated with a drop in cAMP. We suggest that the presence of elevated cAMP levels in the oocyte is directly responsible for the maintenance of meiotic arrest.