It has been demonstrated previously that the Na⁺-K⁺- adenosinetriphosphatase (Na⁺-K⁺-ATPase) concentration (total concentration of [³H]ouabain-binding sites = maximal [³H]ouabain-binding capacity, B(max)) in longissimus dorsi muscle is higher in young pigs living in a cold than in a warm environment, with the effect being even greater in those on a low rather than a high energy intake in the cold. The importance of thyroid status, myofiber type and size, and muscular activity in determining the concentration of Na⁺-K⁺-ATPase has therefore been assessed in longissimus dorsi, semitendinosus, and soleus muscles from 8-wk-old littermate pigs, which had been living 35 or 10°C on either a high or a low energy intake for 3-4 wk. Changes in neither plasma thyroid hormone concentrations nor myofiber type could account for the greater B(max) at 10 vs. 35°C observed in all three muscles; instead, it was possibly related to the greater muscular activity associated with shivering in the cold. More than 50% of the increase in B(max) on the low compared with the high intake in the cold could be attributed to the smaller fiber size and hence greater membrane area in animals on the low intake, and it is concluded that membrane surface area must be assessed in studies of membrane-bound enzymes.