The plasma vectors of thyroid hormones (TH) in trout have been characterized. Plasma components were separated by density gradient ultracentrifugation after first labeling binding sites with trace levels of radioactive hormones, both in vivo and in vitro. Lipoproteins play only a minor role in humans but are major carriers of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in trout plasma. More than 67% of T4 and 89% of T3 were bound to lipoproteins (density less than 1.210 g/ml), predominantly to high-density lipoproteins (HDL), regardless of the nutritional status of the animals. The percentage of hormone bound to very-low-density lipoproteins, on the other hand, was proportional to their concentration and thus to nutritional status. T3 and T4 could also bind to vitellogenin, a very-high-density lipoprotein, which could transfer TH to the yolk of oocytes. Homologous ligand displacement indicated that T3 could bind to at least two classes of saturable sites in the plasma. In addition, plasma HDL were the major binding sites with low affinity (1.7 +/- 0.4 x 10(5) M-1) but with high capacity (3.1 +/- 0.3 x 10(-5) M). In conclusion, these results show that lipoproteins are the principal binding sites of TH in trout plasma.