In addition to their metabolic function of storing triglycerides and releasing free fatty acids upon hormonal stimulation, adipocytes are now recognized as behaving as secretory cells. During the past 10 yr, an increasing number of peptidic and nonpeptidic compounds have been demonstrated to be secreted by adipocytes (1). Most of these compounds are thought to exert primarily, and in an autocrine/paracrine manner, local effects that modulate adipose cell hypertrophy and/or hyperplasia of adipose tissue (AT). This is essentially the case for short-living molecules, such as the prostanoid, prostacyclin (PGI₂), which is discussed in this chapter. Some of these secreted factors, like leptin (see Chapter 26), allow communication with other peripheral tissues and the central nervous system, giving to the adipocyte the status of an endocrine cell. In addition, some of these factors may contribute to the development of morbid complications of obesity, i.e., cardiovascular diseases, hypertension, insulin-resistance, diabetes, and cancer (2).