Plasma thyroxine (T4) concentrations were measured during pregnancy and lactation in goats on diets with different energy concentrations to study the role of T4 secretion in metabolic adaptation to pregnancy and lactation. Thyroxine distribution space (the volume of fluid required to dissolve the total pool of circulating T4 at a concentration equal to that in the plasma) and secretion rates were determined by using 125I-labelled T4 at different stages of lactation and pregnancy. Plasma concentrations of T4 increased during lactation and reached maximum levels 4-5 months after giving birth. They then remained fairly constant for the following 2-3 months until the goats became pregnant again. Thyroxine concentrations decreased slightly during pregnancy and showed a sudden drop at the onset of lactation. Minimum concentrations were observed during the first 2-3 weeks of lactation after which time concentrations increased slowly. Average concentrations for goats on a low-energy diet were consistently lower than those in goats on a high-energy consistently lower than those in goats on a high-energy diet. The mean secretion rate was 10.4 nmol day-1 kg body wt-0.75. The distribution spaces were much lower than the total extracellular space. The average distribution space in non-pregnant and pregnant goats was 11 and 5.3% of the body weight respectively. The decrease in T4 secretion rate and T4 distribution space during pregnancy is considered to be part of a homeorhetic adaptation to the condition. The drop in T4 secretion rate at the onset of lactation appears to be a homeostatic adaptation to a decreased fuel supply, similar to the situation with fasting or energy malnutrition.