Germinal vesicle (GV)-stage horse oocytes with diffuse chromatin are meiotically incompetent and degenerate in culture, whereas horse oocytes having condensed chromatin within the GV are meiotically competent. Degeneration of incompetent oocytes in culture may be related to premature GV breakdown, which could possibly be prevented by inhibition of m-phase protein activity. We examined the effects of 6-dimethylaminopurine (6-DMAP), butyrolactone and roscovitine on GV-stage horse oocytes. Culture in the presence of 2 mM 6-DMAP for 24 h suppressed meiosis (2% MI or MII compared with 38% for untreated oocytes). The proportion of GV-stage oocytes having condensed chromatin was not different between 6-DMAP culture and directly fixed controls; however, the proportion of oocytes with diffuse chromatin was significantly lower, and more oocytes with diffuse chromatin had atypical chromatin than did controls (p < 0.01). Culture with butyrolactone at 100 mM suppressed meiosis (5% MI + II). Again, this treatment maintained GV-stage oocytes having condensed chromatin, but the proportion of oocytes with diffuse chromatin was significantly reduced compared with directly fixed controls (p < 0.05). Culture with roscovitine at 25 μM was also effective in maintaining GV-stage oocytes having condensed chromatin; however, culture with 100 μM roscovitine did not suppress meiosis or maintain oocytes in the GV stage. These results indicate that meiosis in GV-stage horse oocytes having condensed chromatin may be suppressed by inhibitors of m-phase protein activity; however, oocytes originally having diffuse chromatin appear to degenerate in culture even in the presence of these inhibitors.