en Biology & physiology Biology & physiology پژوهشي Orginal research papers Changes in the peptide levels of sea bream (<em>Sparus aurata</em>) GnRH (sbGnRH) in the olfactory bulb, telencephalon including the preoptic area, hypothalamus, optic tectum-thalamus and pituitary during a reproductive cycle were investigated in addition to plasma levels of testosterone (T), 17&beta;- estradiol (E2) and 17&alpha;, 20b-dihydroxy-4-pregnen-3-one (DHP) in wild female Asian sea bass <em>Lates calcarifer</em> in Nha Phu lagoon, Vietnam from March to September, 2010. Ovaries were classified into five stages: II Recovering spent (March -April), III Maturing (May-June), IV Mature (June), V Ripe and ovulating (July- August) and VI Spent (September). The gonadosomatic index increased significantly from the stage II (1.62%&plusmn;0.12) to stage V (8.48%&plusmn;0.23) and dropped thereafter in stage VI (2.05%&plusmn;0.25). In the olfactory bulb and telencephalon, sbGnRH levels in stage IV and stage V were slightly higher than those in the other stages, but no significant differences in sbGnRH levels were found among the ovarian stages. In the hypothalamus and optic tectum-thalamus, although significant differences in sbGnRH level were not found among the ovarian stages, the highest level of sbGnRH was determined in stage III (594.13&plusmn;152.12 pg tissue^-1 and 501.37&plusmn;124.35 pg tissue^-1, respectively). In the pituitary, sbGnRH level in stage III (6927.03&plusmn;218.92 pg tissue^-1) was significantly higher than that in the other stages. In addition, plasma T, E2 and DHP levels increased significantly during ovarian maturation or ovulation with the highest level of T (2.91&plusmn;0.31 ng ml^-1) and E2 (4.98&plusmn;1.43 ng ml^-1) recorded in stage IV and DHP (389.6 &plusmn; 136.03 pg ml^-1) in stage V. The results showed that sbGnRH in the anterior brain and pituitary is involved in initiation and ovarian maturation in wild Asian sea bass. sbGnRH, Brain, Pituitary, Plasma steroid, Ovarian stages, Asian sea bass 3255 3268 http://jifro.ir/browse.php?a_code=A-10-1350-1&slc_lang=en&sid=1 K.X. Pham phamkx@vnio.org.vn 100319475328460038415 100319475328460038415 Yes Institute of Oceanography- Vietnam Academic Institute of Science and Technology (VAST), 01 Cau Da, Nha Trang, Khanh Hoa, Vietnam H.V. Dao 100319475328460038416 100319475328460038416 No Institute of Oceanography- Vietnam Academic Institute of Science and Technology (VAST), 01 Cau Da, Nha Trang, Khanh Hoa, Vietnam M. Amano 100319475328460038417 100319475328460038417 No School of Marine Biosciences, Kitasato University, Sagamihara, Kanagawa 252-0373, Japan.