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Basal metabolic rate and then summit metabolic rate were measured using a water immersion technique in newborn lambs at 1 h or 4 h old. The mean rates of basal and summit metabolism were 51 +/- 1.5 and 235 +/- 7.9 w per m2 respectively. Summit metabolic rate was more closely related to body surface area than to body weight. Umbilical catheters were inserted at birth and blood samples were taken at 5 min of age and during the measurement of metabolic rate to examine changes in acid-base status and energy substrate concentrations. At birth varying degrees of metabolic acidosis and hyperlactaemia were observed. Summit metabolism was associated with the development of a further metabolic acidosis and with increases in the blood levels of lactate and excess lactate and the plasma levels of glucose, glycerol and free fatty acids. The relationships between summit metabolic rate, the metabolic state at birth and the change in metabolic state caused by cooling suggested that prepartum hypoxia was associated with a depression of the thermogenic response to cold stress and also with a depression of sympathetic nervous activity.

The plasma concentrations of adrenaline, noradrenaline, dopamine, thyroxine, tri-iodothyronine, cortisol, insulin and glucose were estimated in lambs aged one to five hours during basal metabolism and then during summit metabolism (the maximum thermogenic response to cold stress). The increase in metabolic rate from basal metabolism to summit metabolism was associated with marked increases in the plasma levels of adrenaline, noradrenaline, cortisol and glucose and a decrease in the plasma level of insulin. There were no quantitative relationships between summit metabolic rate and the plasma levels of any of the hormones measured. These findings suggest that the marked lamb-to-lamb variation in heat production capacity found here and elsewhere cannot be directly attributed to variation in the activities of the sympathetic nervous system, the thyroid, the adrenal cortex or the pancreas. Concurrent estimations of respiratory quotient suggested that the newborn lamb has a marked dependence on carbohydrate as an energy substrate at high rates of heat production.

The feeding of colostrum to newborn lambs led to a 17 to 20 per cent increase in summit metabolic rate estimated at five hours of age. This increase would significantly enhance the survival potential of the newborn lamb in the field. Respiratory quotient during summit metabolism and changes in plasma composition suggested that the carbohydrate content of the colostrum was a major contributor to the increase in summit metabolic rate.

Severe hypoxia during birth results in a metabolic acidosis and depressed heat production capacity. The aetiology of the depressed heat production capacity is unknown. To investigate this problem newborn lambs were exposed to a 30 minute period of hypoxia by causing them to breathe room air diluted with nitrogen. Heat production capacity was subsequently estimated as summit metabolic rate. When summit metabolic rate was estimated at the end of the period of hypoxia a depression to 66 per cent of the control value was observed (P less than 0.01) but no depression was observed when the estimation of summit metabolic rate was delayed for 30 minutes. Hypoxia resulted in metabolic acidosis, hyperlactaemia and considerable increases in the plasma concentrations of adrenaline and noradrenaline. Summit metabolic rate was positively correlated with blood pH at the beginning of summit metabolism in lambs which had a pH value of less than 7.05. There were no other relationships between summit metabolic rate and blood composition. These results suggest that the poor heat production capacity found in lambs which have suffered hypoxia during birth may be directly related to acidaemia.