Summary—The modification induced by interferon (IFN) in brown adipose tissue (BAT) was studied by high spatial resolution magnetic resonance imaging (MRI), histology, and transmission electron microscopy (TEM). In IFN‐treated mice, at MRI, the interscapular BAT was slightly enlarged and showed non‐homogeneous areas of lipid accumulation. The thickness of the subcutaneous white adipose tissue was reduced with respect to control mice. In the liver, MRI showed a lipid accumulation. In IFN‐treated mice, by light microscopy, brown adipocytes showed a larger lipid deposit with respect to control mice. At TEM, in BAT, the mitochondria were reduced in number, smaller and the number of cristae was also significantly reduced with respect to the controls (9.1 ± 1.5 vs20.1 ± 1.9, P<0.01). The inclusions in the mitochondrial matrix were significantly less numerous in IFN‐treated than in control animals (1.9 ± 0.7 vs0.9 ± 0.7 for mitochondrial section, P<0.01). Abnormalities of endoplasmic reticulum described in hepatocytes were not found in brown adipocytes of IFN‐treated mice. The present work demonstrates that, in the BAT of sucking mice, IFN‐treatment induces morphologic alterations and that brown adipocytes have MRI and TEM features resembling those found in the lipid laden BA