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The meiotic competence of canine oocytes cultured for 72 h in medium supplemented with three different concentrations (5, 10 and 20%) of anoestrous, oestrous or metoestrous bitch serum, or with 0.3% bovine serum albumin (BSA), was examined. The oestrous serum supplement had a positive effect on the resumption of meiosis, compared with the other supplements (P<0.05). The number of oocytes that reached metaphase I (MI) and metaphase II (MII) was significantly higher (P<0.05) with the oestrous serum supplement than with the anoestrous serum supplement. There were no significant differences among the three different concentrations in each serum type with respect to the proportion of oocytes that completed meiosis (MI to MII). The number of oocytes that resumed meiosis in the 10% oestrous serum supplement was significantly higher (P<0.05) than those of each concentration of the anoestrous and metoestrous serum supplements, and of the 0.3% BSA supplement. Moreover, a higher number of oocytes reached MII in the presence of the 10% oestrous serum supplement than with the 10% anoestrous serum supplement. These results suggest that supplementation of the culture medium with 10% oestrous serum is the optimal treatment for in vitro maturation of canine oocytes.