Little is known about the action of laser rays on normal adipose cells. The present study attempts to observe the behavior of fatty cells submitted to laser therapy. Dorsal fat pads of normal adult rats were submitted to low-level laser irradiation applied locally through intact skin, with four different dose schedules (4, 8, 12, and 16 J/cm(2)), with a further group being sham-irradiated. Histology, morphometry, immunofluorescence, and electron microscopy were all used to analyze irradiated tissues. Changes were restricted to the brown fatty tissue, in which a tendency was shown for multivacuolar cells to be transformed into the unilocular type. The number of cells which exhibited enlargement and fusion of small vacuoles was greater in the 4- and 16-J/cm(2) groups (p<0.05). Increased vascular proliferation and congestion was another more evident finding in laser-treated animals compared to nontreated animals. Low-level laser rays cause brown adipose fat droplets to coalesce and fuse. Additionally, they stimulated proliferation and congestion of capillaries in the extracellular matrix.