The possible role of hormone-sensitive lipase (HSL) in determining regional differences in lipolysis activation in humans was studied in vitro. Small adipose tissue biopsies were obtained from the abdominal sc and omental regions during surgery in 21 subjects spanning a wide range of body mass index (22-50 kg/m2). In lipolysis experiments, isolated fat cells were incubated with lipolytic agents acting at different levels in the lipolytic cascade. The activity and messenger ribonucleic acid expression of HSL were determined. The maximum lipolytic capacity was higher in sc than in omental fat cells as were HSL activity and messenger ribonucleic acid expression. The maximum lipolysis rate was significantly correlated to HSL activity. This is in accordance with the role of HSL as the rate-limiting step of lipolysis. However, adipocytes were 24% larger in the sc than in the omental region, and the lipolysis rate was significantly correlated to fat cell size regardless of either the region of origin or gender. This indicates that the regulation of HSL activity in healthy subjects, which appears to occur at a transcriptional level, is to a large extent dependent on fat cell size.