We have previously demonstrated that the CCK-B/gastrin receptor ligand CI-988 induces gastric gland degeneration and atrophy in cynomolgus monkeys, an effect consistent with gastrin receptor antagonism and inhibition of gastrin's trophic effects on oxyntic mucosa. However, gastrin receptor ligands of the dipeptoid chemical series to which CI-988 belongs have been reported to act as agonists or antagonists towards gastrin-related events, depending on the animal model and the functional endpoint examined. To investigate further these apparently conflicting data, basal gastric acid secretion was monitored acutely in conscious monkeys given CI-988 orally at 10 mg/kg or intravenously at 0.01 mumol/kg/hr and histological changes in gastric mucosa were evaluated in monkeys given CI-988 orally at 5, 25 or 75 mg/kg/day for 4 weeks. Degeneration and atrophy of gastric glands occurred at 25 and 75 mg/kg with statistically significant decrements in gastric mucosal height at 75 mg/kg. In addition, CI-988 stimulated gastric acid secretion when given either orally or intravenously. Co-administration of the structurally unrelated CCK-B/gastrin antagonist L-365,260 completely blocked CI-988-stimulated acid secretion, confirming that CI-988's agonist effect on acid secretion is mediated by the gastrin receptor. Assuming that gastric mucosal degeneration is the result of inhibition of gastrin's trophic activity, CI-988 appears to induce paradoxical agonist and antagonist gastrin-receptor mediated effects.