This study was conducted to evaluate the effect of caffeine on the meiotic maturation of porcine oocytes. Oocyte-cumulus complexes were collected from slaughterhouse-derived ovaries and cultured for 24, 32 or 48 h in medium 199 supplemented with 10% fetal calf serum, 10 microg/ml FSH, 50 microg/ml sodium pyruvate and 50 microg/ml gentamicin in the presence or absence of 2.5 mM caffeine. Caffeine inhibited the meiotic resumption of pig oocytes effectively after 24 h of culture, and 95.5% of oocytes were arrested at the germinal vesicle (GV) stage (control 17.8%, p < 0.05). Prolonged culture with caffeine up to 32 h or 48 h, however, resulted in a significant decrease in the inhibitory effect (GV: 13.8% and 8.2%). The number of oocytes at metaphase II after 48 h of culture in the presence of caffeine was significantly lower than that in the control medium (65.3% vs 94.7%, p < 0.05). The withdrawal of caffeine after 24 h of culture resulted in the resumption of meiotic maturation, and the oocytes reached metaphase II after 48 h. However, the ability of caffeine-treated oocytes to develop to blastocysts after artificial activation was lower than that of the control (5.5% vs 9.1%, p < 0.05). Caffeine treatment significantly increased cAMP levels in the oocytes after 24 h of culture, while both Cdc2 kinase and MAP kinase activation were inhibited in the oocytes. These results suggest that caffeine, similarly to other purine derivatives, prolongs the meiotic arrest of porcine oocytes at the GV stage, perhaps by its action of increasing the cAMP level and by the suppression of Cdc2 kinase and MAP kinase activities in the oocytes.