Microscopic examination of adipose tissue from lambs of varying ages between birth and 20 days showed that most cells were multivacuolar on the first day of life, but most were univacuolar at 10 days of age and older. The oxygen consumption of four lambs aged between 6 h and [Formula: see text] days was approximately doubled after injection with (−)noradrenaline (400 μg/kg subcutaneously) in an environment of 20 °C. Five lambs between [Formula: see text] days and 12 days showed a reduced response or no response. Three lambs between 14 days and 20 days of age showed virtually no change in oxygen consumption after noradrenaline. It is concluded that the mechanism for nonshivering heat production is present in the newborn lamb, but disappears at a relatively early age.