Uptake of the thyroid hormones (TH) 3,5,3′-triiodo-l-thyronine (T3) andl-thyroxine (T4) by trout red blood cells (RBC) was studied by incubating washed RBC in a balanced salts medium containing glucose and [125I]TH at the fish acclimation temperature of 12°. RBC were separated from the medium by centrifugation through silicone oil and glycine buffer (pH 10.5). Maximal [125I]T3uptake occurred by 10–15 min, but not by 60 min for [125I]T4. First-order uptake was measured at 30 sec for T3and at 90 sec for T4. Total T4uptake was enhanced ≈15-fold from pH 8 to 6 and was affected most below pH 7.2; total T3uptake was maximal between pH 6.4 and 7.0, but was relatively insensitive to pH. At 0.2 nM,nonsaturable uptake of T3exceeded that of T43- to 6-fold, accounting for 3% (T3) and 50% (T4) of total uptake. Saturable TH uptake was described by Michaelis–Menten kinetics. The saturable transport system for T3had an apparentKt(carrier affinity) of 70–119 nMandJmax(maximal uptake velocity) of 540–1116 pmol·106cells−1·min−1. A saturable system was also found for T4, with an apparentKtof 99 pM–1.1 nMandJmaxof 8–77 fmol·106cells−1·min−1. Saturable uptake of both TH depended on temperature. Activation energies for the nonsaturable component were 48 (T4) and 64 (T3)KJ·mol−1over the range 0–21°. Activation energies for the saturable components were 52KJ·mol−1(T4, 0–21°), 52KJ·mol−1(T3, 0–10°), and 3KJ·mol−1(T3, 10–21°). During a 16-month study saturable and nonsaturable uptake of both TH increased, probably due to fish age. We conclude that in trout RBC, rapid T3uptake by a pH- and temperature-sensitive saturable carrier greatly exceeds T4uptake. The rate of T3uptake exceeds by 100- to 1000-fold that of mammals and amphibia, and in contrast to those taxa some saturable T4uptake also occurs.