The thermogenic effect of ephedrine (E) and aminophylline (AP) was investigated in 27 and 20 obese adolescents by indirect calorimetry. Ten children receiving only water served as controls. Nine children from both groups had no thermic response to the drugs (NR). The responders (R) increased their fasting REE by 7.1+ -0.75% and 6.4+ -1.0% after E and AP administration, respectively. After one-week E treatment the thermic effect of the drug disappeared even in the R. One-week AP treatment significantly improved the thermic response in the NR and did not alter it in the R. When the combination of E and AP was given to 8 responders the thermic response was no higher than after the single drug.