Effects of CB-154 therapy on the glucose tolerance, IRI and GH responses to the oral glucose administration in 5 acromegalic patients with impaired glucose tolerance were examined. Glucose tolerance improved in all patients with a reduction in GH levels. Insulin secretion in response to the oral glucose administration was exaggerated in 3 patients, while it was supressed in 2 patients who had severely impaired glucose tolerance. After receiving CB-154 therapy, hyperresponsiveness of insulin observed in the 3 patients and a suppressed response in one patient were corrected to normal, while a low response in another stayed unchanged. This observation suggest that variable abnormalities in insulin secretion induced by the excess of GH improve with a decrease in plasma GH levels. However, the abnormal GH secretion in response to hyperglycemia persisted still after administration of CB-154 in spite of decreased basal plasma GH levels.