Techniques that allow the study of immature subcutaneous depots in newborn and young rats were developed utilizing a histological and histochemical approach. The temporal and spatial relationship between blood vessel development and adipocyte hypertrophy was examined in rats from large and small litters. Large numbers of small (9.8 +/- 1 micron diameter) multilocular adipocytes were present in subcutaneous depots of newborn animals. These adipocytes were intimately associated with capillaries that had lumen diameters of 4–10 microns. Adipocytes rapidly increased in size up to 30–40 microns in diameter. The development of small capillaries (2-4 microns lumen diameters) was associated with this cell hypertrophy. Lipoprotein lipase (LPL) histochemistry and ultrastructural analysis indicated that the larger capillaries may potentiate LPL-mediated cell hypertrophy. Similar analysis indicated that the development of the narrow lumen capillaries may slow the rate of LPL-mediated hypertrophy. These studies indicate that the adipose tissue capillary may play an important role in adipocyte hypertrophy and metabolism.