The interrelationships between body weight, food consumption and plasma thyroid hormone concentrations were studied during the spring, summer and fall in two groups of woodchucks: those allowed to eat ad libitum and those kept at their March weight by food restriction. The food consumption of the ad libitum fed control animals was greatest in April and May and then declined. Body weight in these animals peaked in July and then slowly declined during the rest of the study. When the restricted animals were allowed to eat ad libitum in August they rapidly increased their food consumption and brought their body weight up to that of the controls in less than six weeks. Plasma thyroxine (T4) and triiodothyronine (T3) concentrations decreased during the summer in the control animals and then rose again in the fall. The T4 changes were blunted in the restricted animals. T3 concentrations decreased during the summer in the restricted animals and remained low until one month after they were allowed to eat ad libitum. The low plasma concentrations of T3 during the summer in the woodchuck may facilitate weight gain in preparation for hibernation.