A 33-year female patient with active acromegaly and hyperthyroidism was examined before and after incomplete removal of a pituitary adenoma. Before adenomectomy the mean basal plasma GH level was 786+/-189 ng/ml. After surgery this value decreased to 251+/-22 ng/ml. Before surgery L-dopa decreased the plasma GH levels, but after adenomectomy normal GH responses were transiently observed to L-dopa. At 10 months after incomplete surgery the GH response to L-dopa became abnormal again despite a lower GH concentration. These findings suggest that the abnormal GH response to L-dopa may be due to a short-loop negative feedback system which was reset by an elevated GH level.