The separate effects of ambient temperature and energy intake on the metabolism of thyroid hormones in the young pig have been investigated. Piglets were kept singly from 4 weeks of age at 10 or 35 degrees C on a high (H), or low (L) energy intake, such that H = 2L. At the tenth week of age blood samples were taken 16-21 h after the last meal for the determination of plasma concentrations of thyroxine (T4) and 3,5,3'-triiodothyronine (T3). At the end of the tenth week of age the fractional disappearance rate (K) of T4 was determined and finally the K value for T3 was estimated. Plasma concentrations of T4 and T3 were greater in animals on a high intake than in those on a low intake, (P less than 0.02), but the temperature at which the animals lived had no significant effect. The values of K for T4 and T3 were greater at 10 than at 35 degrees C (P less than 0.010 and P less than 0.05), and energy intake had no significant effect. Calculations of the catabolic rates in nmol/kg X h revealed that for T4 the rate was greater at 10 than at 35 degrees C (P less than 0.001) and on a high than on a low intake (P less than 0.01). Similar differences were found between the mean values of T3 with respect to both temperature and energy intake, but the variance was large and they did not reach statistical significance. It is concluded that the metabolism of thyroid hormones is influenced independently by both ambient temperature and energy intake.