The principal conclusion presented in this review is that no single mechanism underlies any of the examples of basal or altered cellular thermogenesis. Both increased Na+ pump operation and uncoupling may occur to a greater or lesser extent, as may other heat-producing mechanisms. There are areas in which further information is needed in order to explain fully the composite nature of the mechanisms involved in cellular thermogenesis. The control of mitochondrial oxidations in their natural habitat (i.e. inside cells) by regulatory proteins, fatty acids, ions (Ca2+, Na+, K+), cyclic AMP, protein kinases, prostaglandins, purine nucleotides, and other factors must be elucidated. There is evidence for the participation of all of these substances in the control of cellular thermogenesis, but no scheme has been developed that takes them all into account. Further emphasis on the tissue-specific differences in the regulation of mitochondrial function is desirable. The regulation of the biogenesis of mammalian mitochondria is another area currently under intense study for which no clear hypothesis has as yet emerged. Information in this area is needed in order to understand the mechanism and role of mitochondrial adaptations associated with altered thermogenesis in hyperthyroidism, in acclimation to cold, and in exercise training, as well as the nature of altered mitochondrial biogenesis, such as appears to underlie the Luft hypermetabolic syndrome.