The gastric mucosal gland luminal pressure was measured in vivo with a pressure-sensitive microelectrode technique (servo-null) in anesthetized rats. A microelectrode was inserted into a gland lumen by means of a micromanipulator at an angle of 30 degrees to the mucosal surface. Acid secretion was estimated by measuring the pH in the solution covering the mucosa. During control conditions, when the mucosa was secreting acid spontaneously, gland luminal pressure was 12.3 +/- 1.2 mm Hg. At about 9 minutes after starting pentagastrin administration, the luminal pressure stabilized at 17.2 +/- 1.7 mm Hg. In the rats given impromidine (500 micrograms.kg-1.h-1) luminal pressure gradually increased (during 9-10 minutes) from a control level of 9.0 +/- 1.9 to 17.3 +/- 2.6 mm Hg. During the majority of experiments, the luminal pressure oscillated at 3-7 cycles per minute. The results show that intraluminal pressure increases during stimulated acid secretion, indicating that a resistance to the volume secretion exists in the upper part of the gastric crypts. This hydrostatic pressure may well be the driving force for creating channels for acid and pepsin to cross the mucus layer covering the mucosal surface.