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Serum levels of non-esterified fatty acids, free glycerol and esterified fatty acids were estimated in obese patients and in persons of normal weight after intravenous administration of tolbutamide and glucose. In obesity non-esterified fatty acids and glycerol levels were roughly twice as high as in the non-obese, whereas no difference in levels of esterified fatty acids was found between the two groups. Following administration of tolbutamide or glucose, there was a decrease of concentration of non-esterified fatty acids and glycerol to a minimum during the two hours of observation. In obesity, non-esterified fatty acids and glycerol decreased relatively more than in persons of normal weight. This behaviour is interpreted as due to increased insulin secretion in obesity. The difference in changes in concentration of non-esterified fatty acids and glyoerol in response to a variety of test situations may well be the result of both the glucose-dependent lipogenetic and the glucose-independent antilipolytic effect of insulin. The antilipolytic effect of insulin is discussed with reference to the metabolic aspects of obesity.