During the first 3 days of exposure of rats to 5 degrees C, the nitrogen concentration of interscapular brown fat increased by 50% and remained at this elevated level for the duration of the 8-wk observation period, while the mass of tissue increased fourfold. The concentration of both DNA and RNA per unit nitrogen reached a maximum after 3 days, then declined; however, the total quantity of each continued to rise. The concentration of various respiratory enzymes decreased during the first few days and then increased, but at different rates. The morphological changes in mature brown fat cells during cold acclimation were observed to be: a reduction in fat droplet size during the first 3 days, followed by a gradual increase in size through 6 wk in the cold; a continual increase in the amount of intermitochondrial ground substance during the first 3 wk, with increased granularity and glycogen content after 1 wk; initial disappearance of glycogen between mitochondria, followed by the reappearance of a few isolated particles in the intermitochondrial ground substance after 1 wk in the cold; initial increase in the density of intramitochondrial matrix for the first 3-4 days, followed by a gradual return to the control density; loss in integrity of mitochondrial outer membranes during the first 4 days, followed by gradual but incomplete restoration; temporary loss of the dense material in lipid droplets during the first 24 hr, with return after 1 wk in the cold; and a 40% increase in mitochondrial diameter within 1 day, followed by a decrease in diameter within 1 wk to a constant value about 15% larger than the controls.