This study examines the precise time course that brown adipose tissue (BAT) takes to adopt the characteristics of white adipose tissue in postnatal lambs. Perirenal adipose tissue was sampled from ewe-reared lambs within 1 h of birth and at 1, 2, 4, 7, 14, 21 and 30 days of age and analysed for the amount of mRNA for uncoupling protein (UCP), the amount and activity of UCP, and protein, mitochondrial protein and lipid content. This was combined with measurements of colonic temperature and jugular venous plasma concentrations of thyroid hormones and insulin-like growth factor-1 (IGF-1). Over the first 4-7 days of age, large quantities of UCP mRNA were associated with a peak in plasma triiodothyronine concentration at 2 days of age followed by a maximal amount and activity of UCP at 4 days and a basal colonic temperature of 39·3°C. Between 7 and 30 days there was a large increase in lipid deposition as the amount and activity of UCP and the amount of UCP mRNA declined to basal values and colonic temperature was maintained at 40°C. A significant positive relationship between perirenal adipose tissue lipid content and plasma IGF-1 concentration was observed throughout the study period. It is concluded that ovine adipose tissue maturation occurs in two distinct phases over the first month of life. The precise time scale of this process could be regulated in part by the lamb's body temperature which determines whether adipose tissue is required for heat production (i.e. BAT) or as an endogenous energy source (i.e. white adipose tissue).