Prolactin inhibitory tests (bromocriptine in a single dose 2.5 mg orally) as well as stimulatory tests (metoclopramide in a single dose of 10 mg intravenously) were performed in 33 patients with endoscopically proven duodenal ulcer and in 32 healthy volunteers. Delayed response in inhibition of prolactin release as well as in stimulation of prolactin release was observed in duodenal ulcer patients compared to healthy volunteers (P < 0.01). The results implicate the role of the dopamine-prolactin axis in the pathogenesis of duodenal ulcer disease.