The objective of this study is to determine whether abnormal rhythm and amplitude of the oscillation in plasma motilin concentration are responsible for the abnormal motility observed in patients with diabetic gastroparesis. We also investigated the effect of metoclopramide on plasma motilin concentration and gastrointestinal motility in these patients. In healthy controls, basal plasma motilin concentration fluctuated in phase with the interdigestive migrating motor complex. All patients with diabetic gastroparesis did not have antral phase III activities and had significantly higher basal motilin concentrations, which maintained a normal oscillatory pattern. Administration of metoclopramide initiated antral phase III activities in healthy controls and in patients with diabetic gastroparesis. These were accompanied by a concurrent rise in plasma motilin concentration in healthy controls, contrasting with a fall in plasma motilin concentrations in patients with diabetic gastroparesis. We conclude that diabetic gastroparesis is associated with absence of antral interdigestive migrating motor complex and with elevated plasma motilin concentration with normal cyclic fluctuations. Our studies also show that metoclopramide initiates antral phase III activities in diabetic gastroparesis independent of plasma motilin concentration.