The relationships among seasonal differences in body weight, food intake, metabolism, and thyroid hormone in woodchucks were investigated in 12 woodchucks. Six woodchucks had been maintained on a photoperiod simulating that found at 42°N (boreal woodchucks). The other group of six animals had been maintained similarly in all respects except that the light simulated that found at 42°S (austral woodchucks). An open-flow respirometer, calibrated using the N₂-dilution method, was used to determine metabolism twice in a 2-wk period near the September equinox, while at the same time food intake, body weight, and free thyroxine concentrations (fT₄) were measured. Body weight was the same for both groups of woodchucks. However, compared with boreal animals near their autumnal equinox, austral woodchucks near their vernal equinox had significantly higher (P < 0.01) daily food intake (5 ± 2 vs. 35 ± 2 g · kg¹ · day¹), oxygen consumption (4.4 ± 0.3 vs. 7.3 ± 0.3 ml · min¹ · kg¹), carbon dioxide production (2.8 ± 0.2 vs. 6.0 ± 0.2 ml · min¹ · kg¹), respiratory quotient (0.65 ± 0.01 vs. 0.82 ± 0.02), and fT₄ (0.21 ± 0.01 vs. 0.65 ± 0.05 ng/dl). It was concluded that photoperiod has a strong effect on resting metabolism in the woodchuck and that there is an association between fT₄ and changes in food intake and metabolic rate.