The pH of the gut lumen was measured in 39 healthy persons using a pH-sensitive, radiotransmitting capsule. Thirteen persons were studied twice. The location of the capsule was determined by X-ray. The pH rose from 6.4 in the duodenum to 7.3 in the distal part of the small intestine. In 17 persons the pH dropped by 0.1-0.8 pH units during the last hours of the small intestinal transit. The pH was 5.7 in the caecum, but rose to 6.6 in the rectum. Gastric residence time was 1.1 h, small intestinal transit was 8 h, and colonic transit time was 17.5 h (median values). The results provide a firmer basis for prediction of the level, and the rate of release of active substance from pH-dependent sustained-release oral preparations.