After a short introduction (chapter 1) methods of measuring gastrointestinal pH are described in chapter 2. The methods are divided into intubation techniques and tubeless methods, and the advantages and disadvantages are discussed. Measurements with pH-sensitive, radiotransmitting capsules are highlighted, and methodological problems with these capsules are described. Chapter 3 concerns the gastrointestinal pH profile of healthy subjects. The intraluminal pH is rapidly changed from highly acid in the stomach to about pH 6 in the duodenum. The pH gradually increases in the small intestine from pH 6 to about pH 7.4 in the terminal ileum. The pH drops to 5.7 in the caecum, but again gradually increases, reaching pH 6.7 in the rectum. The physiological background of these pH values is discussed. Chapter 4 describes the effect of gastrointestinal pH on bacterial flora, absorption of vitamins and electrolytes, and on the activity of digestive enzymes. The pH-profile in children is described in chapter 5. The profile is identical with that of adults, and it is therefore concluded that the release of a drug from pH-dependent, controlled-release preparations is also probably identical with that of adults. Chapter 6 describes the correlation between certain diseases and the gastrointestinal pH. A resection of the colon and the creation of an ileostomy do not affect the pH of the remaining gut. An ileocaecal resection shortens the small intestinal transit time, increases pH of the proximal colon, but does not change the pH-profile of the small intestine. Chronic pancreatitis and cystic fibrosis seem to decrease pH of the proximal small intestine. Very low colonic pH values have been observed in severe active ulcerative colitis and in Crohn's disease, but the background and clinical implication of this phenomenon are not clear. Chapter 7 describes the modulating effect of diet and drugs on gastrointestinal pH. Diet primarily has an effect on the colonic pH, whereas drugs might affect both small intestinal and colonic pH. The different effects are described. Finally, chapter 8 summarizes the present knowledge about gastrointestinal pH, and future investigations are proposed.