The effects of insulin on the rates of glucose disposal were studied in soleus muscles isolated from hyper‐ or hypothyroid rats. Treatment with triiodothyronine for 5 or 10 days decreased the sensitivity of glycogen synthesis but increased the sensitivity of lactate formation to insulin. The sensitivity of 3‐O methylglucose to insulin was increased only after 10 days of treatment and was accompanied by an increase in the sensitivity of 2‐deoxyglucose phosphorylation; however, 2‐deoxyglucose and glucose 6‐phosphate in response to insulin remained unaltered. In hypothyroidism, insulin‐stimulated rates of 3‐O‐methylglucose transport and 2‐deoxyglucose phosphorylation were decreased; however, at basal levels of insulin, 3‐O‐methylglucose transport was increased, while 2‐deoxyglucose phosphorylation was normal. In these muscles, the sensitivity of lactate formation to insulin was decreased; this defect was improved after incubation of the muscles with prostaglandin E₂. The results suggest: (a) in hyperthyroidism, insulin‐stimulated rates of glucose utilization in muscle to form lactate are increased mainly because of a decrease in glycogen synthesis; when hyperthyroidism progresses in severity, increases in the sensitivity of glucose transport to insulin and in the activity of hexokinase may also be involved; (b) in hypothyroidism, the decrease in insulin‐stimulated rates of glucose utilization is caused by decreased rates of glycolysis; (c) prostaglandins may be involved in the changes in sensitivity of glucose utilization to insulin observed in muscle in altered thyroid states.