One-half of each of two groups of male rats, seventy and two-hundred days of age, were exhaustively exercised five days per week for ten weeks. Thyroid hormone secretion rate (TSR) and serum and hepatic cholesterol and triglycerides (TG) were measured at the termination of the experiment. TSR was effectively increased by exercise in both young and mature animals. In the young, TSR was increased and in the mature, TSR was maintained at a more "youthful" level. The result in both groups was a higher TSR in the exercised animal than non-exercised animals of the same age. The age difference under study was sufficient to results in significant changes in TSR and all lipid classes studied. Exercise lowered lipid levels, but few of the changes were significant. Serum TG and hepatic cholesterol were lowered significantly in the young; while in the mature, only serum cholesterol was significantly decreased. All lipid classes studied were inversely correlated with TSR in the physiological state.