Twelve dogs underwent an extensive proximal small bowel resection. Basal and postprandial serum gastrin levels increased significantly one and two months after the resection compared to the respective preoperative levels. The increase in serum gastrin levels two months after the resection was less prominent than the respective increase one month postoperatively. Antral G- and D-cell populations increased significantly two months after the resection compared to the respective populations before the resection. It is concluded that hypergastrinaemia following small bowel resection is due to the quantitative elimination of the inhibiting hormonal factors of gastric secretion and gastrin release. The adaptive increase of antral D-cell population could explain the temporary character of hypergastrinaemia.