Previous work has indicated that thyroid hormone entry into cells includes an energy-dependent, saturable process. In this study we investigated the effect of insulin on T3 uptake in rat skeletal muscle. Intact soleus muscles were preincubated for 30 minuted at 37 degrees C, pH 7.4, in modified Krebs-Ringer bicarbonate buffer in the presence or absence of insulin and then for 60 minutes after adding 50 pmol/L [125I] T3. The results showed a stimulatory effect of insulin that was half maximal at 33 nmol/L and maximal at 100 nmol/L. Addition of 10 mumol/L T3 to the incubation medium completely blocked the effect, showing that the action of insulin is exerted only on the specific component of T3 uptake. The substitution of extracellular sodium with an equimolar amount of lithium also prevented the insulin effect. [125I] T4 uptake was unaffected by insulin. These results indicate that T3 uptake in skeletal muscle is sensitive to insulin, that insulin stimulates the specific component of T3 uptake in a dose-dependent manner, and that its action requires extracellular sodium. In contrast, T4 uptake is insensitive to insulin action in accordance with previous results that indicated passive diffusion as its major pathway for cell entry.