GLP-1, with its insulinotropic properties and direct action on satiety center in the brain, may be the main hormone regulating the amount of ingested food. In this study, GLP-1 secretion was investigated in age-matched adolescent girls (14 +/- 2 years): 13 with anorexia nervosa (BMI 14.8 +/- 1.4 kg/m(2)), 13 with simple obesity (BMI 33.0 +/- 3.3 kg/m(2)) and 10 healthy girls as a control group (BMI 21.6 +/- 0.7 kg/m(2)). Each girl was subjected to OGTT and standard meal tests after a 12 h overnight fast. Blood samples were collected before and 15, 30, 60, and 120 min after the stimulation. The mean fasted GLP-1 levels in simple obesity group (1.6 +/- 0.3 pmol/l) and in anorexia nervosa group (1.7 +/- 0.3 pmol/l) were significantly lower than those in the control group (2.6 +/- 0.4 pmol/l) (p < 0.05 in both cases). The highest peak concentration of GLP-1 was observed in the control group after both stimuli. In each group, the mean integrated GLP-1 outputs were almost twice as high after OGTT than after the test meal (p < 0.001 in each case). In our opinion, low secretion of GLP-1 in girls with simple obesity may seriously and negatively influence the course of this disease. On the other hand, low GLP-1 levels in girls with anorexia nervosa are beneficial and promote appetite.