The response of plasma growth hormone (GH) to 0.5 g L-dopa was studied in 17 obese nondiabetic subjects and in 6 normal-weight subjects, aged 16 to 46 yr. The test was repeated after the obese subjects were thinned 12 to 50 kg, either with a hypocaloric diet or by a jejunoileal shunt. All the obese subjects had a blunted response to L-dopa stimulation. Nine of the thinned obese continued to exhibit the same blunted GH response, while the other eight responded with a significant GH rise after L-dopa stimulation, although the increase was smaller than that in the controls. No correlation between the initial or final weight or the extent of weight loss and the response to L-dopa was observed. It is assumed that a hypothalamic underresponsiveness is responsible for the blunted response in the obese subjects and its persistence in some of the thinned obese.