About half of calf losses occur during the first two days of life. Mortality rate is increased by difficult parturition and adverse climatic conditions. However, thermoregulatory mechanisms are operative at birth, these including the metabolism of brown adipose tissue, shivering and physical activity. Thermoregulation is just as effective in twin as in single calves provided that calving conditions are good. Heat production of Charolais and Salers calves was lower than that of Friesian; the difference came from a lower basal metabolic rate rather from a better thermic insulation. In dystocial calves, blood pH at birth was very low, lactataemia was two or three times higher than in eutocial calves, mobilization of body lipids was reduced and plasma thyroïd hormone level were low, which can explain the lower heat production and the drop in rectal temperature. The physiological responses of calves born by caesarean parturition depend on the delay incurred during surgical removal.