Surface pH of rat intestine was measured in vivo in exteriorised loops using pH-electrodes. Surface pH was approximately 6.1 in Krebs-phosphate buffer in proximal jejunum and was significantly more acid (p less than 0.01) than the bulk medium pH of 7.2. Values for the midgut were approximately 6.5, yet the distal ileum gave values of 7.3, which was marginally more alkaline than the buffer. These results agree with the known acidification phenomenon in the jejunum and the alkalinisation process in the ileum. No difference in surface pH was detected when bicarbonate buffer was used, nor when glucose was included in the buffer. The acid surface pH was almost completely inhibited when jejunal loops were made anoxic by completely occluding the blood supply, showing that the low surface pH is not itself a consequence of preparative anoxia. If glucose was included in the buffer, allowing glucose access from the luminal surface, surface pH was not significantly altered after occlusion. This indicates that previously reported in vitro results in the presence of glucose are similar to the present in vivo findings. The present experiments confirm the existence in vivo of the 'acid-microclimate' proposed to alter the absorption profiles of some dissociable drugs.