NE OF THE MOST important functions of adipose tissue is to provide for the temporary storage of absorbed food energy and for the release of ths energy 0 into the blood stream for transport to body cells. The stored lipid energy of adipose tissue is mainly in the form of triglycerides in the fat cells (adipocytes). The triglycerides can undergo lipolysis and the resulting glycerol and free fatty acids may pass out of the fat cells into the blood stream. During fasting and in other conditions of increased physiological demand of the body for energy, large quantities of free fatty acids pass out from the fat cells, become loosely bound to albumin in the plasma and travel in the circulating blood as a fatty acidalbumin complex to muscle, liver and other tissues, for oxidative degradation yielding some 9 kilcalories per gram on complete oxidation to carbon dioxide and water. In the liver some of the free fatty acids are partially oxidised to ketone bodies and are released into the circulation for use by muscles and other tissues (Fig. 1). Fatty acids mobilised in excess of physiological needs may be temporarily stored in the liver but are eventually repackaged and returned to the adipose tissue as plasma lipoproteins. The glycerol fraction released from adipose tissue may serve as a substrate for gluconeogenesis in the her, and in fasting subjects may account for as much as 10 per cent of the daily glucose production. Because of the tremendous importance to the organism for a rapid and adequate supply of free fatty acids from the adipose tissue stores to meet energy needs of all the body cells (with the possible exception of the cnetral nervous system, medulla of the kidneys, red cells, and lens of the eye), especially during fasting, in the post absorptive state, during muscular activity and cold exposure, several mechanisms can be brought into play to ensure adequate mobilisation of stored lipid from adipose tissue into the circulation. Lipolysis of triglycerides in the fat cells and their release into the blood stream are under various controlling mechanisms which modulate the net eflux of free fatty acids to suit the differing requirements of body cells for energy. Several hormones, the sympathetic nervous system, availability of metabolic substances and various drugs, can modify these processes. The effects they produce vary with animal species, nutritional status, the cell size and the site of the adipose tissue, age, pathological conditions and other factors. The more important aspects of adipose tissue metabolism including the processes of lipolysis and fat mobilisation have been described in a recent article (Pawan 1971). The fat mobilising substance, FMS, has been detected in the urine of man and some other animal species during fasting, carbohydrate deprivation, and in several other conditions associated with increased fat catabolism (Chalmers, Kekwick, Pawan and Smith 1958; Chalmers, Pawan and Kekwick, 1960; Karvinen, Miettinen * Based on a paper read to the London Branch of the British Dietetic Association, 26 May, 1971, 165