The respective roles of sex steroids and hormones related to growth and metabolism, on SBP regulation have been studied in rainbow trout. In vivo, oestradiol (E2) supplementation induces a slow but significant increase of plasma SBP concentration. Testosterone or cortisol injections have no effect. In vitro, the steroid binding protein that accumulates in incubation medium of hepatic cell primary cultures has been characterized and found to be similar to blood SBP. Its production is increased by addition of E2 (maximum: +300%). This effect develops slowly over several days of culture and is dose dependent; as little as 1-10 nM E2 is effective. Recombinant rainbow trout GH (rtGH)--0.01 to 1 microgram/ml--also increases SBP accumulation as compared to control cells and seems to maintain SBP production over culture duration. In preliminary experiments, (1) insulin-like growth factor (IGF) and SBP concentrations were found to change inversely after a 4 days stimulation with increasing concentrations of GH; (2) recombinant human IGF1 (250 ng/ml) tended to be inhibitory when SBP production was expressed per mg of total cellular protein, and a micromolar concentration of bovine insulin was clearly inhibitory. Other hormones tested in vitro: triiodothyronine (10-1000 nM), thyroxine (100 nM), 17 alpha, 20 beta-dihydroprogesterone (10-2000 nM), and testosterone (1-1000 nM) did not influence SBP concentration in hepatic cells culture media.