The effect of fetal thyroidectomy on thermoregulation in newborn lambs was investigated. Seven of 14 lambs born normally at term were thyroidectomized at Day 127 of gestation. Colonic temperature and oxygen consumption were measured during non-rapid eye movement sleep 6-45 h after birth. All lambs were then killed and perirenal brown adipose tissue was sampled for measurement of thermogenic activity (guanosine diphosphate binding), uncoupling protein and lipid contents. Thyroidectomized lambs tended to have a mean colonic temperature 2.35 degrees C lower (P = 0.067) than controls and two became hypothermic (i.e. colonic temperature < 35 degrees C). Thyroidectomized lambs exhibited lower rates of oxygen consumption (P = 0.05) and an increased incidence of shivering thermogenesis. The perirenal adipose tissue of these lambs had a lower thermogenic activity (P < 0.01), less uncoupling protein (P < 0.01) and higher lipid content (P = 0.072) compared with intact controls. It is concluded that fetal thyroidectomy results in a decreased ability of newborn lambs to utilize nonshivering thermogenesis in brown adipose tissue as well as increasing the incidence of hypothermia. These changes are associated with decreased synthesis of uncoupling protein and functional development of brown adipose tissue in the late gestation fetus.