The administration of a diet, containing 70 cal% of fructose, increases in rats, as compared with a diet containing 70 cal% of glucose, the release of free fatty acids (FFA) from adipose tissue in vitro. Since the glycerol release from adipose tissue is not increased by the high fructose diet, it may be concluded that the enhanced FFA release in the fructose group is due to a reduced re-esterification of FFA in adipose tissue rather than to an enhanced hpolysis. The serum concentrations of FFA are increased by fructose intake temporarily (for the first week of the diet administration), the increased FFA release from adipose tissue being present up to 1 month after starting the diet. The reduced re-esterification of FFA in fructose-fed rats is probably connected with the reduced glucose utilization in adipose tissue. None of the described effects of the fructose diet seems to result from insulin deficiency since no difference was found in the serum insulin concentrations between the groups on the fructose and glucose diet. It is speculated whether the enhanced FFA release from adipose tissue in rats fed the fructose diet may participate in the mechanism of fructose-induced hypertriglyceridaemia.