The plasma-gastrin response to feeding and to insulin stimulation was determined before and after precise antrectomy in conscious rhesus monkeys (Macaca mulatta). Feeding resulted in a significant increase above basal gastrin levels (107 plus or minus 13 to 276 plus or minus 22 pg. per milliliter), as did stimulation with insulin (93 plus or minus 8 to 182 plus or minus 23 pg. per milliliter). Antrectomy reduced basal gastrin concentrations and abolished the gastrin response to both feeding and insulin. Antrectomy abolished the acid secretory response to histamine and insulin. In order to eliminate the problems of reflux gastritis and to study the time course of reduced acid secretory capacity, three monkeys were antrectomized and maintained by duodenal feedings. The histamine acid secretory response virtually was abolished within 24 hours and was not recovered over a 4 week period. Parietal cell architecture remained intact. In the monkey a major portion of basal circulating gastrin is antral in origin and the gastrin response to feeding and insulin is from the antrum exclusively. Loss of antral gastrin results in a virtual abolition of the acid secretory response to insulin and histamine. This decline is immediate in onset and is not related to parietal cell atrophy.