Male Badgers were shown to display an annual body weight (BW) cycle characterized by an autumnal phase of fattening, coinciding with depressed plasma levels of thyroxine, and a phase of emaciation starting in late winter and leading to a minimal BW in summer, when plasma thyroxine concentrations were peaking. Surgical thyroidectomy in the fall did not suppress the BW cycle, but the onsets of fattening and of emaciation were advanced, respectively by 1 and 2 months. On the other hand thyroxine supplementation in late winter through spring, enhanced the seasonal BW loss. The possible interaction of thyroid hormones with other lipolytic factors in the control of the BW cycle is discussed.