In this study, we provide evidence that plasminogen activator of tissue-type (t-PA), at least, is present in extracts of bovine oocyte cortical granules, and that its activity varies significantly with the duration of oocyte in vitro maturation. Cortical granules were collected from bovine oocytes by means of micromanipulation, after 0, 12, or 24 h of IVM. Our results show that plasminogen activator activity of cortical granule extracts was significantly higher after 24 h of IVM than after 12 h of IVM or before IVM. This activity was apparently due, at least partly, to tissue-type plasminogen activator as shown immunologically. No evidence was found for the presence of urokinase-type plasminogen activator, plasminogen activator inhibitors or plasmin inhibitors in bovine oocyte cortical granule extracts. Our findings further support the hypothesis that t-PA activity of oocyte origin may have a role in oocyte maturation or fertilization, as well as in post-fertilization events, such as cortical reaction and formation of the zona block to polyspermy.