The effect of truncal and proximal gastric vagotomies on the secretin secretion was investigated on 28 dogs. Truncal vagotomy was applied 15, and proximal gastric vagotomy on the rest of them. The secretin secretion was stimulated giving HCl of 0, 1 N into duodenum. The level of secretin in portal plasma was measured with RIA at 3, 5, 10, 15, 30 and 45th minutes just before and after stimulation. It was found out that various responses were given to the stimulation when the results were obtained before vagotomy, just after it and one month later. After truncal vagotomy the response of taken out duodenal acidification decreased significatively. A clear increase was noticed in the response one month later, but this increase didn't reach to the level seen just before truncal vagotomy. The responses in proximal gastric vagotomized group were not effected by the operation in early stage, one month later a decrease was seen in the response, the later stages.