The participation of the monocarboxylic acid transporter MCT1 in the intestinal absorption of weak organic acids has been clarified by functional characterization, by use of stably transfected cells, and by immunohistochemical location of the transporter in intestinal tissues. Immunohistochemical analysis by use of the anti-MCT1 antibody showed that MCT1 is distributed throughout the upper and lower intestines, especially in the basolateral membrane and, to a lesser extent, in the brush-border membrane. When the transporter gene rat MCT1 was transfected into MDA-MB231 cells, transport of benzoic acid, a model weak organic acid that has been generally believed to be transported across the cell membranes by passive diffusion, and lactic acid in rat MCT1-transfected cells was significantly increased compared with transport in cells transfected with the expression vector pRc-CMV alone (mock cells). The observed transport was pH-dependent and activity increased between pH 7.5 and pH 5.5, whereas pH-dependence in mock cells was moderate. Rat MCT1-mediated benzoic acid uptake was saturable, with an apparent Km value of 3.05 mM. In addition, MCT1 increased the efflux of [14C]benzoic acid from the cells. Several weak organic acids were also transported by rat MCT1. These results show that pH-dependent intestinal absorption of weak organic acids, previously explained in terms of passive diffusion according to the pH-partition hypothesis, is at least partially accounted for by MCT1-mediated transport energized at acidic pH by utilization of the proton gradient as a driving force.