Peptic secretion was studied in fasting anaesthetized cats in which the pylorus and common bile duct had been occluded to prevent the release of duodenal hormones which might stimulate or inhibit gastric secretion. Dilute acid was instilled into the stomach at intervals to aid recovery of gastric secretion and to preserve peptic activity. 2. Caerulein, histamine and N-methyl histamine did not increase the output of pepsin when given on their own. Desulphated caerulein was a weak peptic stimulant. 3. Two C.H.R. u./kg per hour secretin initiated pancreatic secretion, the volume of which increased progressively as the dose was increased by stages to 32 C.H.R. u./kg per hour. 4. Four C.H.R. u./kg per hour secretin did not increase the output of pepsin. Peptic secretion was stimulated by 8 C.H.R. u./kg per hour. A maximal output of approximately 2000 u. pepsin/15 min was obtained when 16 C.H.R. u./kg per hour was infused. 5. When each acid stimulant was infused along with 4 C.H.R. u./kg per hour secretin the output of pepsin increased significantly. The peak output, which usually occurred between 15 and 30 min after stimulation, did not exceed 1000 u. pepsin/15 min. 6. The proposed explanation for the potentiation of the peptic response when an acid stimulant is infused along with a dose of secretin, in itself below the threshold of peptic stimulation, is that each acid stimulant increases gastric mucosal blood flow, approximately doubling the effective concentration of secretin delivered to the peptic cell.