Adipose tissue is known to consist of at least 2 compartments, the adipocytes and the non-lipid filled cells. During normal growth and development of the rat epididymal fat pad, these 2 compartments changed in different manners. From 12 to 35 days of age, the DNA contained in both compartments increased linearly, indicative of hyperplastic growth. From 35 to 70 days of age, the DNA in the non-lipid filled cells continued to increase linearly; DNA in the adipocyte fraction increased more slowly. From 70 to 182 days of age, DNA accretion continued in the non-lipid filled cells while remaining unchanged in adipocytes. From 35 to 70 days of age, an abrupt change in the rate of tissue lipid accumulation occurred, shown both by a tripling of fat cell size and a markedly increased slope in the accumulation of lipid per pad. These data confirm that adipose tissue growth proceeds as suggested by radioactive thymidine incorporation studies and further suggest that a critical period for the onset of lipid filling may begin around 35 days of age.