Adipose tissue in rats maintained at normal ambient temperature grows by a mixture of cell proliferation and lipid deposition in the early growth stage of the rat. In the mature rat, the tissue grows primarily by lipid deposition, mitotic activity being significantly decreased. When the rat is acclimated to 5 C, growth of adipose tissue is less than that of rats maintained at normal ambient temperature. Furthermore, growth of adipose tissue in the 5 C rat occurs through a mixture of cell proliferation and lipid deposition throughout the body weight range studied. The differences in tissue growth were taken into consideration in measuring the stimulatory effect of norepinephrine on lipolysis and reesterification of isolated fat cells. The results indicate that the cell size affects the lipolytic response; the larger the cell the less sensitive it is. Fat cells from cold‐acclimated rats are more sensitive to the lipolytic action of norepinephrine, independent of differences in cell size. On the other hand, reesterification is not affected by cell size, nor by exposure of the rat to cold

Adipose tissue in rats maintained at normal ambient temperature grows by a mixture of hyperplasia and hypertrophy in the early growth stage of the rat. In the mature rat, the tissue grows primarily by hypertrophy. When the rat is acclimated to 5 C, growth of adipose tissue occurs primarily by hyperplasia throughout the body weight ranged studied. As a consequence adipose tissue from rats maintained in the cold has a much larger number of fat cells which are much smaller in size. In order to achieve the adaptation which permits hyperplasia to continue throughout the adult stage the animal must be exposed to cold at an early age, and it requires that the animal remain in the cold to continue. These changes in adipose tissue fat cell size and number are discussed in relation to the increased calorigenic and lipolytic effect of norepinephrine in cold‐acclimated rats.

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