Skeletal muscles are most often examined at the cellular level in relationship to their primary role in force generation. Throughout the animal kingdom, regardless of phylogeny, muscle generates heat. Exercise, shivering, and nonshivering thermogenesis provide excess heat in muscle that affords adaptive significance to a wide variety of organisms. Although there is reasonable concordance on the mechanisms involved in muscle as a force generating cell, the physiological mechanisms for thermogenesis, biological significance, and evolutionary role of muscle as a heat-producing cell are not as clearly defined. Heat liberation resulting from contractile activity is well understood, actively studied, and has been carefully measured by muscle energeticists. Heat production during periods of nonshivering thermogenesis (NST) is not as well defined, and its existence is controversial in many species, despite compelling evidence for NST from fish, birds, and mam mals. Investigators interested in the role of heat production in skeletal muscle primarily focus on the relationship between heat liberation during the contraction-relaxation cycle (1, 77, 90, 1 10, 128). Discussions of heat production independent of contractile activity usually focus on mammals where extensive reviews exist (44, 46, 47, 83, 86, 95). Despite major efforts to understand how muscle contributes to basal metabolic rate (BMR) and to cold-induced thermogenesis in endotherms, the mechanistic basis of such activity remains poorly defined. The fundamental mechanisms for heat generation without contractile activity have not been elucidated. This review examines skeletal muscle in light of its secondary role as a furnace and