1. Short-chain fatty acid absorption in hen colon is protonated across the apical border coupled to an apical electrogenic proton pump. 2. The surface pH of the isolated colonic epithelium was 6.27 +/- 0.05, when incubated in Krebs-phosphate buffer pH 7.0. 3. Propionate 7 and 40 mmol/l in the incubation medium (pH 7.0) increased microclimate pH to 6.47 +/- 0.04 and 6.56 +/- 0.04. Inhibition of metabolic activity by potassium cyanide 1 mmol/l increased surface pH to 6.66 +/- 0.06. 4. The calculated concentration of propionic acid in the microclimate is near-linearly related to the propionate concentration. Thus, the acid microclimate is not responsible for the Michaelis-Menten like kinetics of propionate transport.