In contrast to fully grown oocytes, growing mouse oocytes are not capable of undergoing germinal vesicle breakdown (GVB) when released from the follicle unless they are first cultured in somatic-cell-conditioned medium. The first objective of this study was to assess the mechanisms by which oocytes in vitro acquire the ability to resume meiosis in conditioned medium. Whereas most of the denuded oocytes that were initially incompetent of undergoing GVD underwent GVB within 4 days of culture in fibroblast-conditioned medium, oocytes cultured in control medium remained in the GV stage although their viability was sustained as judged by morphological appearance and quantitative and qualitative patterns of protein synthesis. This suggested that the effect of somatic-cell-conditioned medium is inductive rather than simply permissive. When GVB-incompetent oocytes were first incubated in control medium for 1-3 days, a larger percentage underwent GVB following exposure to the conditioned medium or okadaic acid. It was therefore concluded that some aspects of the oocytes' developmental program for the acquisition of GVB competence are oocyte-autonomous but external factors provided by the surrounding somatic cells are probably require for oocytes to become fully GVB-competent. The effect of cAMP on acquisition of GVB competence by growing oocytes was also studied. Dibutyryl cAMP dose-dependently promoted the acquisition of GVB competence by initially GVB-incompetent oocytes. Forskolin, an adenylate cyclase activator, acted in similar way and its effect was potentiated by hypoxanthine, a naturally occurring cAMP-phosphodiesterase inhibitor. Thus cAMP, in addition to maintaining meiotic arrest in GVB-competent oocytes, also participates in the acquisition of GVB competence by growing oocytes.