The purposes of the present study were to examine the effect of naloxone, a mu-opioid receptor (MOR) antagonist, on porcine oocyte maturation and embryo development. MOR gene was expressed in germinal vesicle (GV) and metaphase II (M-II) porcine oocytes, one-, four-cell stage embryos and blastocysts. In blastocysts, MOR gene was mainly expressed in inner cell mass (ICM) cells. Supplementation of 10(-8) mol/L naloxone in in vitro maturation (IVM) medium increased the maturation rate (P < 0.05). However, 10(-4) mol/L naloxone reduced the maturation rate (P < 0.05) compared with the control. The presence of naloxone during IVM had no effects on fertilization status and subsequent embryonic development after in vitro culture (IVC). The addition of 10(-3) mol/L dibutyryl cyclic adenosine monophosphate (dbcAMP), and 10(-8 ) mol/L naloxone together into IVM medium increased nuclear maturation (P < 0.05) compared with the addition of either dbcAMP or naloxone alone. Supplementation with naloxone in IVC medium did not improve embryonic development. However, at the concentrations of 10(-6) mol/L and 10(-8) mol/L, naloxone increased the ratio of ICM to total cells in blastocysts (P < 0.05). In conclusion, at low concentration, naloxone increases maturation rate and the ratio of ICM to total cells in blastocysts. Naloxone and cAMP have a synergistic effect on oocyte maturation.