The influence of different experimental and physiological conditions on sex steroid binding protein (SBP) concentrations in the blood (and in hepatic and testicular cytosols) has been studied on male rainbow trout. SBP was measured with a specific binding assay. The aim was to further understanding of regulation of the SBP and in particular to determine the respective influences of reproductive and metabolic status. Twelve days after hypophysectomy, pituitary and steroid hormones were dramatically decreased and SBP concentrations were significantly lowered in blood (-32%) and in hepatic cytosol (-46%) while the binding protein concentration remained constant in testicular cytosol. Castration of maturing animals did not influence SBP concentration in blood and liver cytosol. Toward the end of the reproductive cycle, a dramatic decrease (-80%) of plasma SBP concentration occurred that appears independent of androgen changes that take place during this period, but is concomitant with a rapid increase of plasma growth hormone (GH) levels (and possibly secondary to a natural arrest of food intake). Long-term fasting that increases endogenous GH but decreases plasma IGFs (insulin-like growth factors) concentrations also induces a limited but significant decrease in SBP concentration. Treatment of intact control trout with recombinant bovine GH (1 microgram/g wt, twice a week, for 6 weeks) increased plasma IGF concentrations but did not significantly increase SBP levels. These results suggest that in mature male trout, testicular androgens have little or no influence on SBP regulation. GH levels or liver GH responsiveness and IGF might be involved in SBP regulation. This would, in part, explain the large decrease in SBP around spermiation in trout.