Growth hormone (GH) acts on adipose tissue by accelerating fat expenditure, preventing triglyceride accumulation, and facilitating lipid mobilization. To investigate whether GH is involved in the development and metabolism of interscapular brown adipose tissue (BAT), a site of nonshivering thermogenesis, we employed three lines of transgenic mice. Two of the lines are dwarf due to expression of a GH antagonist (GHA) or disruption of the GH receptor/binding-protein gene. A third mouse line is giant due to overexpression of a bovine GH (bGH) transgene. We have found that the body weights of those animals are proportional to their body lengths at 10 weeks of age. However, GHA dwarf mice tend to catch up with the nontransgenic (NT) littermates in body weight but not in body length at 52 weeks of age. The increase of body mass index (BMI) for GHA mice accelerates rapidly relative to controls as a function of age. We have also observed that BAT in both dwarf mouse lines but not in giant mice is enlarged in contrast to nontransgenic littermates. This enlargement occurs as a function of age. Northern analysis suggests that BAT can be a GH-responsive tissue because GHR/BP mRNAs were found there. Finally, the level of uncoupling protein-1 (UCP1) RNA was found to be higher in dwarf mice and lower in giant animals relative to controls, suggesting that GH-mediated signaling may negatively regulate UCP1 gene expression in BAT.