Tumor necrosis factor-α (TNFα) is a cytokine with multiple biological functions which, in mammals, has been shown to modulate muscle and adipose tissue metabolism. In fish, TNFα has been identified in several species. However, few studies have examined the role of TNFα in fish outside the immune system. In this study, we assessed the effects of human recombinant TNFα and conditioned media from rainbow trout lipopolysaccharide (LPS)stimulated macrophages (LPS-MCM) on lipolysis in isolated rainbow trout adipocytes. Furthermore, we studied the effects of an LPS injection in vivo on lipid metabolism. In our study, human recombinant TNFα stimulated lipolysis in trout adipocytes in a time- and dose-dependent manner. Similarly, LPS-MCM stimulated lipolysis in trout adipocytes when compared with control conditioned medium. Experiments using specific inhibitors of the MAP kinase pathway showed that p44/42 and p38 are partially involved in the lipolytic effects of TNFα. On the other hand, adipocytes from LPS-injected rainbow trout showed higher basal lipolysis than adipocytes from control fish after 24 h, while this effect was not seen at 72 h. Furthermore, lipoprotein lipase (LPL) activity in adipose tissue of LPSinjected fish was lower than in the controls at 24 h. These data suggest that TNFα plays an important role in the control of lipid metabolism in rainbow trout by stimulating lipolysis in vitro and in vivo and by down-regulating LPL activity of adipose tissue in vivo. © 2005 Society for Endocrinology.