An x-ray method modified by the authors was employed in 40 patients with peptic ulcer of the duodenum to examine the effect of pentagastrin and histamine on the rate of gastric evacuation of x-ray capsules 3, 5 and 10 mm in diameter taken with the trial breakfast. It was established that pentagastrin inhibits gastric evacuation (P less than 0.05) mainly at the expense of retention of the egress from it of the capsules 10 mm in diameter (P less than 0.05). Administration of histamine does not exert any essential effect on the rate of gastric evacuation. Administration of gastrin following cimetidine also brings about an inhibition of gastric evacuation (P less than 0.05) largely at the expense of retention of evacuation of the capsules larger in diameter. It is concluded that inhibition of gastric evacuation after gastrin administration occurs at the expense of an elevation of the tone of the pyloric sphincter. It is suggested that in the mechanism of the inhibitory action of pentagastrin, the level of hydrochloric acid secretion and H2 receptors of histamine do not play any important role. The inhibitory effect of pentagastrin is likely to be caused by an elevation of the somatostatin level.