We have studied the effect of agents known to stimulate adenylate cyclase on spontaneous meiotic resumption in vitro by mouse oocytes. We have found that cholera toxin (CT) (up to 1 microM) and prostaglandin E1 (PGE1) (up to 160 microM) are not able to prevent meiotic resumption, but a clear dose-dependent delay in meiosis resumption was observed during the first 3 h of incubation in medium containing CT or PGE1. The effect became clearer when a small concentration of isobutylmethylxanthine (MIX) (1 microM) was added to the medium. We have also measured the cAMP content of the oocyte: the basal content is 2.1 fmol; this value drops to 0.9 fmol during a 2-h culture period. The decrease is partially prevented if CT is present in the incubation medium, while total cAMP content increases to 3.1 fmol in the presence of 1 mM MIX. The results suggest that isolated mouse oocytes contain toxin and prostaglandin sensitive adenylate cyclase and also an active phosphodiesterase system.