1. The 'fade' phenomenon seen in acid gastric secretion in response to a continuous intravenous infusion of pentagastrin has been studied in anaesthetized cats. Whereas acid secretion to a continuous infusion of histamine steadily increased for 3 hr, that to pentagastrin infusion rose steeply to a maximum at 45 min and then decreased gradually to reach about 60% of maximum after 2 hr. 2. Pre-treatment of the cats with the 5-hydroxytryptamine (5-HT) blocking drugs, methysergide and cyproheptadine or with the 5-hydroxytryptamine-depleting drug reserpine completely altered the pattern of secretion to pentagastrin such that the response was similar to that seen during an infusion of histamine. These drugs did not affect secretion stimulated by histamine. 3. It is suggested from these results that pentagastrin releases 5-hydroxytryptamine which at first potentiates and then inhibits the resulting stimulated secretion.