By means of a dose-response secretion test the sensitivity of gastric acid secretion was investigated in 85 patients with chronic renal failure and in 85 age- and sex-matched controls. The renal patients were also gastroscoped, with biopsy specimens taken from the gastric body. The examinations were repeated on 18 patients undergoing regular dialysis and 8 patients after successful transplantation. The acid secretion sensitivity of the stomach among the non-dialyzed patients was decreased when compared with the controls (p < 0.01) but tended to normalize during the intermittent dialysis treatment (p < 0.05) and particularly after transplantation (p < 0.01). The low secretion responses were independent of gastric body histology and were also seen in patients with normal body mucosa. The maximum theoretic acid output did not differ significantly from that of the controls. It is concluded that there is an inhibition of gastric acid secretion in chronic renal failure. This inhibition depends on the decreased sensitivity to stimulation and is diminished by treatment of renal failure by dialysis or transplantation.