Metabolic rate and tissue blood flow were measured by indirect calorimetry and the radioactive microsphere technique, respectively, in anaesthetized pigs aged 10, 17, and 28 days during infusions of saline or noradrenaline. Noradrenaline produced a rise in oxygen consumption in all pigs but the magnitude of the response declined with age in Large White pigs (percent increase: 10 days old, 29.5 +/- 3.3; 17 days old, 15.3 +/- 3.2; 28 days old, 5.4 +/- 0.17) and was less in 10-day-old Minipigs (9.1 +/- 5.1). Regional blood flow per gram of tissue was similar for pigs of all ages and noradrenaline produced increases in flow to heart and skin, and small reductions in blood flow to skeletal muscle. Noradrenaline increased average blood flow to adipose tissue (four sites) by 21-fold in 10-day-old Large White pigs, but had less effect in 10-day-old Minipigs (9.5-fold) and in older Large White pigs (9-fold), where the effect on metabolic rate was diminished. It is calculated that adipose tissue could be responsible for the increase in metabolic rate during infusion of noradrenaline. Macroscopically there are no gross differences between various adipose tissue depots in the pig but those which showed the greatest response to noradrenaline correspond to areas where brown adipocytes have previously been identified by electron microscopy.