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 ug/ml FSH, 50 jig/ml sodium pyruvate and 50 [ig/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 (65.370 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%).