We describe a simple method for the collection of stool water using dialysis membrane tubing placed directly into the feces. When the membrane was placed in pure aqueous solutions or in homogenized feces stored at 4 degrees C, equilibration for solutes was complete in 24 h. Exchangeable ions, short chain fatty acids, osmolality, and pH were measured in the fecal water of 8 normal subjects and 36 patients affected by different diarrheal diseases. The composition of fecal water collected by this method was almost identical to that obtained by ultrafiltration of feces immediately after collection. Differences, when present, were minimal (about 4%) and practically irrelevant. This in vitro dialysis method is simple, inexpensive, and seems particularly suitable for the measurement of osmolality, electrolytes, and short chain fatty acid concentrations in large numbers of fecal samples.