Extracts of the proximal third of the duodenum from 8 antrectomized dogs with the gastrointestinal continuity restored by gastroduodenostomy (n = 4) or gastrojejunostomy (n = 4) as well as from 4 unoperated controls were subjected to gel chromatography. The eluates were all assayed using two different gastrin antisera, one directed against the COOH-terminal end of gastrin and the other directed against the NH2-terminal end of gastrin-17. The gastrin component pattern was very similar in all antrectomized dogs regardless if they had a gastroduodenostomy or a gastrojejunostomy. Gastrin-17 was found to dominate while the amount of gastrin-34 was at most one tenth of that of gastrin-17. Using the COOH-terminal directed antiserum approximately 15% (mean value) of the total gastrin-like immunoreactivity eluted in a peak appearing in the same region as the COOH-terminal octapeptide of cholecystokinin. In unoperated control dogs the corresponding peak constituted approximately 70% (mean value) of the total gastrin-like immunoreactivity. In two of the control dogs small amounts of gastrin-like immunoreactivity appeared at the elution volumes of gastrin-34 and gastrin-17. In the duodenal extracts of all dogs gastrin-like immunoreactivity was found between the elution sites of gastrin-34 and gastrin-17. This material probably represents cholecystokinin-33. The present results show that the increase in duodenal gastrin found after antrectomy, which we have reported previously, is due mainly to an increase in gastrin-17.