Changes are described in the total pyruvate dehydrogenase (PDH) activity, the proportion of PDH in the active state and its control by insulin and noradrenaline in vivo, in white adipose tissue, liver, skeletal muscle and mammary gland with pregnancy, lactation and on weaning. Lactation resulted in a decrease in total PDH in white adipose tissue and an increase in the mammary gland, whereas the proportion in the active state decreased in muscle and increased in the mammary gland. The ability of insulin to activate PDH of white adipose tissue was lost during lactation, whereas it was retained by the other tissues. The ability of noradrenaline to activate PDH was decreased in white adipose tissue but increased in liver during lactation. These various adaptations should limit the use of glucose and lactate carbon by adipose tissue and skeletal muscle during lactation and thereby facilitate their preferential utilization by the mammary gland.