Radioautographs of the interscapular brown fat of six weeks old animals showed that within 24 hours after cold exposure at 4°–5°C, proliferation increased in endothelium of the already profuse capillary network and also that of arterioles and especially that of venules. This continued at a high level for 48 hours then declined. Extravascular fat precursor cells but not fat cells showed an increased proliferative activity at 48 hours cold exposure which continued until 96 hours. By eight days, adaptation to the cold occurred and proliferative activity was not increased thereafter nor did it occur significantly in animals seven months old. Animals exposed to cold 48 hours, given thymidine-H3, and then returned to a temperature of 23°C for a week or more showed a considerable number of labeled fat cells which presumably were precursor cells when the thymidine-H3 was given. When the interscapular brown fat was denervated on one side, there was no loss of lipid nor increased proliferative activity in the vascular or extravascular cells on that side after 48 hours cold exposure. The normally innervated opposite side, however, had more proliferating cells than usual, especially in the endothelium.