The development of adipose tissue lipoprotein lipase (LPL) activity was determined cytochemically in sections from normal and decapitated pig fetuses. Cellular LPL activity was first evident in 60-day-old fetuses. Capillaries were stained for LPL activity in 85-day-old and older fetuses and in young pigs. Cellular synthesis of LPL and capillary staining were not concurrent events. LPL activity developed after adipocyte differentiation and after structural development of capillaries. The cellular and capillary staining of fat cell clusters throughout development and in decapitated fetuses indicated a possible inverse relationship between cellular synthesis of LPL and capability of capillaries to transport LPL to luminal sides. These observations indicate that LPL activity develops in a pattern quite distinct from that observed in vitro. This discrepancy may be due to the involvement of blood vessels in the in vivo developmental pattern.