We have determined the serum gastrin response to a standard test meal in 13 unoperated patients with combined fasting hypergastrinaemia (more than 150 ng/l) and basal hypersecretion of gastric acid (BAO more than 10 mmol/h). In 10 of 11 patients with proved of presumptive Zollinger-Ellison syndrome the peak postprandial rise in serum gastrin concentration was less than 50 per cent of basal value. The integrated gastrin response to feeding ranged from 71-9 to 211-8 mug/lX 150 min. In 2 patients with proved hypergastrinaemia of antral origin, however, serum gastrin rose to 223 per cent and 255 per cent respectively of basal value after ingestion of the meal. The integrated postprandial serum gastrin responses in these patients were 66-8 and 22-0 mug/lX 150 min. Two patiets with Zollinger-Ellison syndrome and total gastrectomy showed peak serum gastrin levels after feeding of 174 and 255 per cent of basal concentration. The integrated postprandial gastrin secretions were 365 and 366 mug/l X 150 min respectively. It is concluded that the serum gastrin response to feeding, when expressed as percentage change, may be helpful in the differential diagnosis of unoperated patients with fasting hypergastrinaemia and basal gastric acid hypersecretion.