The role of the interscapular brown adipose tissue (IBAT) in the defense against cold exposure and subsequent acclimation to it was studied in the rat, a homeotherm. When placed at 4 degrees C, the rat was able to maintain a constant core temperature even though skin and even more so tail temperature dropped drastically. Metabolic rate became significantly higher than control animals kept at 23 degrees C after 24 hrs and continued to rise throughout the 6 weeks exposure period. Starting at week 1, there was hypertrophy of the IBAT with concomitant proliferation of mitochondria. Bilateral surgical removal of IBAT led to significant reduction in metabolic rate only in the fully cold-acclimated rat. However, the survival rate was not affected. It is thus concluded that IBAT played an important but not absolutely essential role in cold acclimation.