Intravenous administration of caerulein (40 ng/kg) delayed gastric emptying of solids in man. Radiological examinations demonstrated that the peptide causes a marked contraction of the pyloric sphincter and a relaxation of the gastric corpus and fundus. Therefore the effect of caerulein on gastric emptying may be considered as the result of different actions on the distal and proximal stomach. Furthermore pretreatment of the subjects with cimetidine did not modify the effect of caerulein indicating an action independent of the gastric hypersecretory effect of the peptide. The action of caerulein on gastric emptying must be kept in mind when the peptide is employed in the treatment of some pathological conditions.