The micro-opioid receptor (MOR) was identified in equine oocytes, cumulus and granulosa cells. By RT-PCR, a 441bp fragment was observed. By immunoblotting, a 65 kDa band was detected in samples of winter anestrous whereas in cells recovered in breeding season, two bands, 65 and 50 kDa, were found. The 65 kDa band was significantly more intense in winter anestrous specimens. In samples recovered in the breeding season, this band significantly decreased with the raise of follicle size and was heavier in compact oocytes and cumulus cells. The protein was localized on the oolemma and within the cytoplasm of oocytes and cumulus cells. In vitro oocyte maturation rate (MR), analyzed by confocal microscopy for nuclear chromatin, microfilaments and microtubules, was reduced after the addition of 3 x 10(-8) M beta-endorphin in medium without additional hormones. Inhibitory effects of 10(-3) M Naloxone in oocytes collected in anestrous and spring transition were observed, both in presence and absence of hormones added to culture medium. Increased MRs were observed in oocytes collected in anestrous and cultured in presence of 10(-8) M Naloxone. The exposure to 10(-3) M Naloxone induced significant intracellular calcium increases in cumulus cells recovered all over the year. beta-Endorphin 3 x 10(-8) M induced significant calcium increases only in cumulus cells recovered in fall transition and anestrous. Naloxone 10(-8) M did not induce intracellular calcium modifications. We conclude that the MOR is differentially expressed in equine cumulus-oocyte complexes in the different seasons of the year and plays a role in the seasonal regulation of meiotic competence of equine oocytes.