Sex hormone-binding globulin (SHBG) transports androgens and estrogens in the blood of vertebrate species, including fish, and regulates the bioavailability and metabolic clearance of these steroids. Liver is the major site of plasma SHBG synthesis, while an SHBG homologue, known as the androgen-binding protein, is produced in testes. When shbg gene expression was examined throughout European sea bass (Dicentrarchus labrax L.) development, SHBG mRNA was clearly detectable at 7 days post-fertilization and persisted throughout embryonic development. In male and female sea bass, the liver is the principal site of shbg gene expression, as determined by SHBG mRNA analyses. Immunoreactive SHBG is present in the liver and villous stroma of the intestine in both sexes. It is also present in the interstitial space between testicular lobules, and the connective tissue surrounding the ovary in the non-reproductive season and around post-vitellogenic oocytes. Plasma SHBG levels were measured over a 10-month period as male sea bass undergo sexual maturation. Immature females of the same age were also studied over the same time interval. The mean+/-S.E.M. plasma SHBG levels in 2-year-old males and females are lower (80+/-15nM and 82+/-16nM, respectively) during the winter reproductive season (December-March) than the spring (April-June) months (144+/-32nM and 193+/-18nM, respectively). In both sexes, plasma SHBG levels start to decline 1-2 months before the reproductive season, coincident with a period of rapid weight gain, while increases after the reproductive season are not accompanied by significant changes in body weight. In addition, plasma SHBG in triploid (sterile) and diploid (fertile) male sea bass do not differ during the first spawning season. These data suggest that the decrease in plasma SHBG levels during sexual maturation in sea bass is related to nutritional or metabolic effects in relation to water temperatures and food intake, rather than changes in gonadal sex steroid production.