This study examined whether the connexin (Cx) is an essential protein during oocyte maturation in the ovary of the goldfish (Carassius auratus). In mature female goldfish ovaries, at late vitellogenic stage, human insulin-like growth factor-I (IGF-I; 20 M) and human chorionic gonadotropin(hCG; 20 IU/㎖) were injected. Twelve hr after the injection, mature female goldfish ovaries were removed and stored at -80C until analysis by RT-PCR. From the goldfish Cx43 cDNA sequence (GenBank accession number AB078505), two degenerate primers were designated. In vivo, 12 hr after the treatment with hCG, goldfish Cx43 mRNA expression level was increased, while the levels of IGF-I was not changed. Goldfish Cx43 mRNA expressed after, but not before the hCG treatment. These results suggest that Cx43 mRNA was judged to be a gene, which was transcribed during oocyte maturation induced by hCG