Adaptive thermogenesis is one component of energy expenditure and occurs in brown adipose tissue (BAT) and skeletal muscle. Recent clinical studies show that BAT activity is reduced in obese subjects. This reduction in thermogenesis may contribute to impaired weight loss. This study aimed to characterize the effects of diet-induced changes in body weight and thermogenesis on thermogenesis in fat and muscle tissues. Ovariectomized ewes were made obese (79±3.7kg; n=4) by feeding high energy supplements or made lean (32±1.5kg; n=5) by dietary restriction; differences in body weight were maintained for 1 year. To measure thermogenesis, temperature probes (Dataloggers) were implanted into the sternal and retroperitoneal adipose tissue and skeletal muscle of the hind limb. Animals were meal fed (1100-1600h) to entrain post-prandial thermogenesis. In lean animals, baseline thermogenesis was reduced in skeletal muscle (P=0.002), sternal fat (P=0.01) and retroperitoneal fat (P<0.05), but this effect was less pronounced in the latter tissue. Tissue temperatures were similar in the control and obese animals. In the muscle of lean animals, reduced thermogenesis was associated with lowered uncoupling protein (UCP) 1 and UCP3 gene expression, as well as altered mitochondrial function with an increase in the respiratory control ratio and a reduction in total respiratory capacity. Sarco/endoplasmic reticulum calcium-dependent ATPase (SERCA1) expression was also lowered indicating impaired calcium cycling. In sternal adipose tissue of lean animals, UCP3 protein levels were lowered but there was no effect of body weight on UCP1 expression. Nor was there any effect of altered body weight on UCP1 or UCP3 expression in retroperitoneal fat. In conclusion, reduced adiposity lowers thermogenesis in a site-specific manner, which may be a compensatory mechanism.  Our data suggest that, in sheep, adaptive changes in thermogenesis due to low body weight primarily occur in sternal adipose tissue and skeletal muscle.