Considering the significance that pH value could have for digestive and absorptive processes, these investigations were aimed at precisely localizing the position of the acid microclimate, i.e., of proton accumulation along the surface of intestinal villi. The determinations were carried out under microscopic control on jejunal segments of rats incubated at 25 degrees C in O2-saturated phosphate buffer (pH 7.4). Specially manufactured antimony microelectrodes (tip diam 50 microns) and calomel reference electrodes were used for pH registration. Highest proton concentration (214-224 nmol/l not equal to pH 6.67-6.65) was found 10-100 microns below the tip of the villus in the zone of digestive and absorptive epithelial cells. Toward the crypt, a steep decrease of proton concentration was registered with alkaline values 200 microns below the villus tip. Toward the bulk phase, the decrease of the proton concentration was moderate due to the existence of the unstirred water layer as an effective diffusion barrier. The pH value of the bulk phase was reached 440 microns over the villus tip, a distance possibly identical to the thickness of the unstirred water layer.